It's more of a trouble than I thought. I had been getting a wifi connection in the house, but now that's gone. I'm getting NO wifi connections at all at this point. The devices say they're connected, but they don't have an IP address in my LAN's address space - not even one in ANY LAN address space.

Could this be a bug from an auto-update over the past day or two?

  @ImaginaryTango 

You have two Deco mesh systems, both run in AP mode, both on the same LAN. I don't even know if it is supported configuration. I can't figure why would you do that. 

It is possible some of issues you have seen is because of that. I also have M9 Plus in my Deco mesh, it takes very little time for mesh to come back after I power Deco units off and on.

I would recommend to delete "Barn" Deco mesh, bring Deco units back to house, add them to "House" mesh and then take them back to barn. You will have single Deco mesh with single Main Deco running in AP mode. See How to delete a network on the Deco app

To find what hardware version(s) your Deco M9 Plus have, login to Main Deco with browser and check Firmware Upgrade section, example:

You might need to wait a little till it populates, and/or click on "Check for upgrades" button to populate it. In this example, there are two M5 with hardware V3 (listed as 3.0) and two M9Plus with hardware V2 (listed as 2.0).

Firmware 1.2.12 is very old. This may explain some of the issues you see with your Deco mesh. It is so old it can't be upgraded through Deco app. You will need to do it manually, follow this document: How to Update the Firmware of Deco

If your Decos have hardware V2 use Method 3, otherwise - Method 2.

  @Alexandre. 

1) Thanks - I'm going to do the upgrade through the web interface later this afternoon, when I have time to do it all at once and undisturbed.

2) Regarding multiple meshes on the same LAN: There are reasons (from experience) I have two SSIDs (and meshes), but that's a longer story. As for Deco being able to handle it, with 2 SSIDs, the app it adds an icon to the SSID that, when touched, provides a list of my meshe SSIDs, so it's clearly made to handle more than one mesh per network. It's been a year or two that this has been working without issue, so I can't imagine that something I've been doing all along is suddenly an issue - unless it went bad just after an upgrade or other change. I'm sure if the app doesn't just check the LAN and grab the first mesh it sees, but, instead, looks and sees them all and can list them, that it's designed to support multiple meshes. (I can imagine a number of scenarios where this would happen, like in business incubators or maker spaces where there might be multiple groups sharing the same space or in an office building where the offices are in close proximity.)

3) My impression of the Deco system is that as long as everything is stable, it works fine, but that it's not fault tolerant (and if my firmware is that old, then I realize things might have changed a lot over the past 3 years. When I started, I ordered 5 units, 2 for the house, and 3 for the barn. I set up the mesh in the house first and took time to test it before setting up the one in the barn. While just one mesh was operating, we had a power outage. We have a standby generator that, at the time, only covered the 1st floor of the house. The main node was downstairs and the other node was upstairs. When power goes out, it can take up to 50 seconds before the generator is providing full power. (And this is conditioned and so on - everything to make sure the power works for everything in the house.) At this point I did not have UPS's on all the electronics. (Now I do - small capacity ones that can easily provide current for a few minutes, bridging from power loss to generator power.) So when this outage happened, power went out for both Deco units, then, in about 45 seconds, came on for the main unit. The other one stayed off. The main unit would NOT work (and I believe the LED stayed red) until I ran an extension cord to the second unit upstairs. Even then, it took more than 20 minutes for the two to sync again and start working. (And, if it matters, as long as we have power, we have internet.) That was a major frustration (until, eventually, I got a competent electrician who could wire in the proper transfer switch so the generator provides power for both floors of the house and barn). The main node would just not work properly without the other node and it would take a long time to sync and restart them.

When I set up the Deco system in the barn, it was a problem, too. The barn is an old pig barn. The first floor is a 25' x 50' box with a concrete floor and cinder block walls, so I don't expect an inside wifi signal to go far beyond the cinder block walls, but it would bounce around inside. The 2nd floor is right above the 1st. When I set it up, I put the main Deco almost dead center of the 1st floor and put the 2nd node up on one end of the 2nd floor, about 35' (in a straight line) from the main unit. I had trouble adding it and plugged it into the ethernet connector I had. It still took about an hour to get the two connected and for the 2nd node to start working as a wifi provider on the mesh. I tried starting it up and going through the steps needed and nothing happening. I waited for over 5 minutes and finally restarted both units. As I mentioned, it took over an hour of trying things like this to get the two to talk to each other, even when they were on the same LAN, connected through a TP-Link switch. Adding the 3rd unit was just as frustrating.

Now? Whatever happened in the barn is now happening in the house. My wife got up this morning  to no wifi in the house.

So, overall, my experiend with Deco is that IF it's set up and has time to work things out, it works, but once anything creates an issue, it falls apart. I'm not exactly impressed with it at this point, but I'm willing to see what happens when I upgrade the firmware.

  @ImaginaryTango 

You can use Deco app to create and manage multiple Deco meshes, but it is for WiFi mesh at your house and, for example, at cottage. Not for running two Deco meshes in AP mode on the same LAN.

Even if app does allow you to do something like that, there is no guarantee Deco mesh will work correctly and I do believe that half of the issues and headaches you have with Deco is because you configured them in unusual way. 

Here is my setup. I have two M9 Plus and two M5s in my Deco mesh, for almost 3 years. Main Deco M9 Plus. I run Deco mesh in AP mode. In these 3 years I had at least two power outages at my house. I bounced Satellites and Main during my testing of Deco mesh. I upgraded firmware more than once and it required restart.

The system is stable, it recovers promptly after every restart. I have no issues with WiFi, in fact complains from family about WiFi issues stopped after I deployed Deco mesh. Had it like that for three years.

I like to tinker with networking gear and troubleshoot it, but my Deco mesh leaves me bored. One of the reasons I come here and try helping others - my own Deco mesh runs just fine.

My recommendation stays: have just one Deco mesh at your house and barn. Either that, or have your Deco mesh(es) in a state that is perfectly described by the subject of your original post. 

Here are other recommendations I am going to make:

1. Upgrade all Deco units. For Deco V2.8 firmware of V2.0 should work, according to description from Support page:  (Normally Vx.0=Vx.6/Vx.8 (eg:V1.0=V1.6/V1.8).

Try Method 2 or Method 3  or even firmware recovery process to push latest firmware to it. Deco app will tell you that 1.2 firmware is the latest instead of telling it can't upgrade that firmware. This is Deco app issue. The latest firmware for V2.0 is in fact 1.6.2.

After Deco unit upgraded to 1.5/1.6 firmware manually, Deco app can be able to upgrade it when new firmware is released by TP-Link. You are only going through the manual process once.

2. After you've upgraded all Deco units, in Deco Main Network security settings, under Security, make sure it is WPA2 (not just WPA).

3. When you add new Deco unit to the mesh as Satellite, it should not be hardwired. This is not properly documented and appears it caused you issues when adding Deco units to your mesh. The correct process is to bring new unit to where Main Deco is, and let them communicate over WiFi while you are going through the process of adding Deco unit to the mesh. After it is added successfully, this is when you can take it to its permanent place and hardwire with Ethernet cable (or have it communicate with Main Deco over WiFi).

  @Alexandre. 

I'd love to upgrade the three hardware V2.8 Decos, but I can't. Can't do it in the app and even after I dismantled the 2nd mesh, and tried using the web interface, but no web interface comes up for them. I even resorted to giving them assigned IP addresses, but the 2.8 Decos don't show a web interface. (The common factor is the version - this happened with the 2nd mesh, without it, with one added to the original mesh and no 2nd mesh - the common factor is the V2.8. The second 1.1 (the label says 1.1, the app says 1.0) unit does show a web interface, even when it's part of the House (original) mesh).

Interesting that a 2nd node should not be hardwired. As you pointed out, that's not documented (at least anywhere that would make it easily found!). That's also frustrating. Say a Deco has a range with a radius of X. Then the 2nd unit has to be within X to extend the range. That makes the range from one end to the other 3X, but if you can use ethernet cable to connect them, then the coverage is closer to 4X. (And if you have 3 units, you can nestle the 3rd in to fill in part of that gap and still extend out almost 2X to the side.)

I have given considerable thought to using one mesh, even though there would be a gap of several hundred feet between each end, but I'm trying to just re-establish the 2nd one first. (Which I've been able to do today.) There are multiple reasons why life is easier using 2 meshes. (including filtering, especially if I do use a fixed IP address for at least one mesh. The barn is where visitors, guests, and business contacts come to, so that's the network I would soon like to have some restrictions on.)

------------------------------------

I still have to disagree on the issue about two meshes on the same LAN. If I had been running into continual issues, or had not gone a couple years without an issue, I'd be more open to that. It's also not just that the app can setup multiple meshes, but that it doesn't not register only the one mesh that already exists, but, even when there is a mesh, allows you to still add one and that it provides listings of multiple meshes. The app has been out for at least 3 years (well, I've been using my system for not quite 3 1/2 years) and that would lave more than enough time to have added in simple features like preventing creating a 2nd mesh when it sees the first mesh. Speaking as a retired programmer, it took extra work in coding that app to make it read more than one mesh in one location, and on one LAN. I would find it odd that any business would pay programmers to write code that specifically allows users to do what the company doesn't want them to do. If it's a problem, then, at the very least, when adding a mesh, it would be trivial to determine if that mesh is on the same LAN and to warn users, "Don't do this, it could cause trouble." So if it's a problem, and they're not providing that warning (and they are not) AND to have added extra functionality to read 2 meshes on a LAN, I can add that as another issue with this system

Also, please note that the worst issues I've had with startup were before I added a 2nd mesh. It was during a summer when we were having an number of heavy thunderstorms - even a 700 year rain event over where we live in that time period. We had several outages, some were flickers. That 45 second outage until the generator kicked in was always followed by a good length of time (at least 5, but sometimes over 20 minutes) before wifi came up again. If I recall, there were 3-4 outages in that time. (That's what led me to get a UPS specifically for the outlet where the main node for the house mesh was plugged in. Usually I only added a UPS for the critical outlets- like where a computer, printer, and several other devices were plugged in, or for the entertainment center, but the wifi outage issue was bad enough I put a UPS on that outlet for only one device.)

Boring is often good and, as I've mentioned, as long as both meshes are constant and nothing unusual happens, it just stays stable. But when something different happens, Deco, at least the earlier version I have, can't handle it.

Another note on multiple meshes on the same LAN, sharing the same address space:

I caleld TP-Link tech support about this. The first person did not understand a lot of basic concepts. He kept asking about my internet connection and I finally said, "I have a firewall with the internet on one side and the LAN on the other. For this discussion, you don't have to know how I'm connected, just that the internet is on the other side of my firewall, and that my firewall is also the DHCP server for the LAN." He said, "What do you mean 'firewall' and what do you mean 'DHCP?'" He did not understand and kept asking for more details about the internet connection. My ISP is Starlink and my dish is connected to the house by 1,000' of fiber optic cable. He had already shown that he didn't understand you can use fiber optic cable and, to the computers on the LAN, it looks like ethernet. It took me 15 minutes to convince him to get me to a Level 2 tech.

When the Level 2 guy called me back, he asked intelligent questions, like, "When you talk about your firewall, do you mean a physical device, and not something in the modem? And is it connected to your main network by an ethernet cable?" I said it ran into a TP-Link multi-port switch. He asked, "Managed or unmanaged?" In other words, this Level 2 tech had a high level of technical knowlege and understood me. He asked a lot of good questions, like, "So when you say fiber optic cable, you mean you have an ethernet cable coming out of the switch, to a converter, to the fiber, then, on the other end, another converter and is that also connected to an unmanaged switch in that building?" I was SO relieved that he was asking questions that showed he wanted a clear picture of the situation and they showed me he really got what I was doing.

So once he understood my setup, he said there was no problem having different mesh networks on my LAN, even in the same address space. He said, "Just use the 'Add Network' button in the Deco app to do that." In fact, he was more concerned about the firewall and verifying that there was nothing going on with the firewall that prevented both mesh networks connecting to the internet, then went into how that can be a problem.

To summarize what the Level 2 tech said, two meshes in the same address space, on the same network, is not a problem and the app is designed to be able to set those up. His concern was more about making sure my routing on my firewall did not have any rules or anything that would block a 2nd mesh network.