@ImaginaryTango 

What is the topology of the Deco, and what is connected to any port.

What are the LED light indicators when the issue happened?

What is the mode you are running the Deco in? (Router vs AP Mode)

@Tony I'm not sure what you mean about the topology - do different versions have different shapes?

When this happens, of course, the 2nd Deco is off - no power at all.  The main one, when this happens, has a red LED (solid red).

On the main Deco, it is connected to power and, via CAT5e, to my central hub (also by TP-Link).

Under normal considtions, the 2nd one has a power connection and a CAT5e connection to the hub as well.  But that also has a short CAT5 cable in the 2nd RJ45 port that connects to a Sonos speaker.

It's in AP Mode.  (I can't remember if I tried it in Router mode.  I doubt it, though.)

@ImaginaryTango 

Use the Deco app to verify if the main unit is the one turning red or the power that stays powered off. I think what is happening is the main Deco is the one staying off. If so, designate the other Deco as the main Deco, that way if the second Deco stays off the other Deco doesn't care.

@Tony I've verified multiple times, including checking which is on and off and making sure the two units are labeled properly.  The one staying on, with power, and with the red LED, is the main one.

@ImaginaryTango 

So the way the Deco system works is if the main Deco is off, then the rest of the satellite units will turn red.

Now in your case if the satellite is the one turning off, and the main is turning red, then the main Deco is telling you the it is no longer getting an internet connection. So from the Deco that is on (main) to the router, something is not communicating.

Another way to test it would be to switch the Deco to router mode and have the satellite node just with the power cable plugged in. If you were to power down the main Deco, the satellite will turn red. However, if the main Deco was left one, and you power down the satellite unit, the main will still be green and functioning.

@Tony I'm not ignoring this, but have been backed up with more immediate issues that are creating a lot of stress and taking up a lot of time.

There is one thing that may help in ID'ing the units. I have been able to use the Deco app on my phone to verify that the downstairs unit is the main unit. I've confirmed that at times by checking which devices are on the main and satellite devices. The ones near the 2nd floor Deco show up on the satellite unit and the ones downstairs and near the end of the house where the main unit is show up on main.

When I lose power, I still have internet, so for the main unit to throw an error due to no internet is a problem. (And, honestly, even if I don't have internet, I want wifi working, since our TV goes through a DVR program on a local server and I have a media server with music and close to 8TB of video that we can use if there is no internet - but if wifi is out, then that has to be accessed by Ethernet.

@ImaginaryTango hey there - I was reading this because I'm interested in other people's experiences with the M9 - so am I correct from reading this that your "Main" Deco remains on solid power, and it's a "Satellite" Deco in the Library that loses power for 30-40 seconds, and that brings your whole Wi-Fi down (even though the "Main" Deco never loses power or internet?

Couple of questions:

Is the "Satellite" Deco connected via ethernet to the main Deco directly or through a switch/hub? I am wondering if it's something to do with the Decos automatically sensing which one is plugged into a router versus a switch?

@ImaginaryTango 

If the main Deco loses internet, you don't want anything wired to it as the Deco then does not know what to do via the LAN ports or what is to give it an internet connection, so to avoid that you want to wire whichever to the satellite units instead. That way, you should still have local connectivity.

@Tony The two units are wired together via ethernet, yes, through a hub, but, again, the issues appear when the satellite loses power. And being out in a rural area, I do have internet failures. One time such a failure went on for over a week.

It sounds like the Deco is so fancy it can't provide functionality unless it has all functions availble. I lose the satellite and it goes down, even though, according to TP-Link, it shouldn't. Lose internet and it goes bad - which, to be honest, is inexcusable, since it still has the LAN DNS server (which is on the firewall/internet gateway).

@ImaginaryTango 

Similar situation here, configured as AP, all connected via Ethernet as well, but if one of the satellites goes down due to lack of power, the whole WiFi network is down for a very long time. Recovery is very slow. If the main deco goes down, the satellites keep working (ethernet) ... seems like it needs a firmware update ...