@DimiFox 

This is not behaving as expected.
The devices connected on the Deco should be capable of communicating with each others when the connection to Internet is lost.

You could verify that by unplugging the Ethernet cable linking the main Deco to Internet.

That should not affect the LAN IP addresses, nor any LAN connection.

  @yves_b Last time I was up at night and noticed, that the cable modem after some period of time without internet connection started rebooting again and again. The reboot was running until the connection was restored on the provider side and the modem caught that. Can these reboots affect the LAN, causing the troubles, that I have described in the original post? I guess the modem sends some signals over the connected LAN cable to the router upon each reboot and I'm curious if that causes the LAN restart as well? What's going to happen if we start the routers without internet connection? Will it still get the required IP range, the devices, etc., or since the DNS and other things are down it won't work after restart? It is very possible, that I'm facing this issue. In this case the router needs a new feature - to ignore the restart/reset commands over LAN.

  @DimiFox 

Is the Deco configured to operate in Router or in AP mode?

Are all devices connected to the Deco, none to the cable modem?

How did you determine that the LAN was not operational?  Did you ping one device from another device? Was the wifi network down?  What specific facts did you observe?

The Deco work well when restarted with no access to Internet.

  @yves_b 

\> Is the Deco configured to operate in Router or in AP mode?

It's a mesh with 3 devices, the main one is connected to the cable modem. The entire network has its own IP range, etc. One sattelite is connected via cable, another one through WiFi.

\> Are all devices connected to the Deco, none to the cable modem?

Of course, non of them is connected to the modem.

\> How did you determine that the LAN was not operational?  Did you ping one device from another device? Was the wifi network down?  What specific facts did you observe?

the devices were unreachable, IPCONFIG showed no usual LAN IP addresses. I can run some tests little later, hopefully today (it's December 31st, remember? :), and post the results here.

\> The Deco work well when restarted with no access to Internet.

Good to know, thanks. I will need to run some tests and will post the results here. So far I'm not satisfied with how it works with no internet. Logically it should work anyway since I use static IP's inside LAN to connect to my NAS'es, but even them were unreachable. The laptop and both NAS are connected through a single 10 GBit switch and they were still unreachable by their static IP addresses. And finally, I'm not sure why, but my tp-link 10 GBit switch TL-SX-105 had to be cold restarted after the internet was recovered, which is absolutely strange. It seems it got absolutely incorrect IP addresses and got stuck so I had to restart it and let it catch the right LAN parameters. Other switches, and I got a few more, all are 1GBit ones, were ok.

  @DimiFox 

In the "advanced" setting in the app, you can see if the Operation Mode of the Deco mesh is Router or AP.

You have switches as well in your configuration, how are they connected?  No switch between your main Deco and the cable modem?

How are the static IP addresses assigned?  Hardcoded on the device, or via DHCP reservation (on the modem or on the Deco?) ?

I am surprised that some devices lost their IP addresses.

  @yves_b 

\> In the "advanced" setting in the app, you can see if the Operation Mode of the Deco mesh is Router or AP.

Of course it's Router.

\> You have switches as well in your configuration, how are they connected?  No switch between your main Deco and the cable modem?

Of course not, the main Deco is connected directly to the cable modem. There is a gigabit switch between the primary Deco and the secondary one. The third Deco is WIFI connected to the primary Deco. There is a switch, connected to the 3rd Deco and several devices, including players, TV, PS, 3 (currently) cameras. There is a plan, although, to run a cable to the 3rd Deco, but it works as it is now.

\> How are the static IP addresses assigned?  Hardcoded on the device, or via DHCP reservation (on the modem or on the Deco?) ?

As DHCP reservation, based on the MAC address of every client. In some unique cases couple devices are having the IP addresses set directly in these devices, but it's rather an exception.

\> I am surprised that some devices lost their IP addresses.

Definitely need to run more specific tests, like diconnected modem, or connected to LAN modem and disconnected from the cable, etc.

  @DimiFox 

As the Deco is configured to act as a Router, the DHCP server of the Deco should be permanently active, and, as it is permanently reachable by all LAN devices.

You mention devices with no usual LAN IP addresses.  I expect those unusual addresses were either assigned by the device (169.254 range) or assigned by another DHCP server (not by the Deco).

My assumption:
From what I read, the main suspect is your 10 GBit switch. 

Could it start acting as a DHCP server when it detects it is isolated from Internet?
Or, after a loss of power for the whole home, could it start acting as a DHCP server because it does not detect any active DHCP server?

  @yves_b 

\> My assumption:
\> From what I read, the main suspect is your 10 GBit switch. 

\> Could it start acting as a DHCP server when it detects it is isolated from Internet?
\> Or, after a loss of power for the whole home, could it start acting as a DHCP server because it does not detect any active DHCP server?

Thanks, I can check these ideas, but...

1) It's just an unmanaged switch, cannot work as DHCP server for sure.

2) There is no power loss, all devices, including routers, modem, cameras and switched are sitting on dofferent APC UPS'es

But I'd check my 10 GBit switch, since it started acting pretty unusually. In the worst case scenario I can redirect the cameras missing this switch, either through 1GBit switch or from the PoE switch directly to NAS, which is less convenient, since I won't get direct access to the cameras, but at least there is something to play with.

  @DimiFox 

This is a known limitation of the Deco's if the WAN connection stops the whole device goes offline, searching for the WAN signal ie No LAN connectivity, no DHCP No dns etc. - brain dead, but a known issue see here:

WAN/LAN auto-sensing - Home Network Community (tp-link.com) Many many posts about this issue.

I had to buy a diff product as this has still not bee resolved. I have Deco X50's I hope to use them one day when there is a good firmware upgrade.

\> This is a known limitation of the Deco's if the WAN connection stops the whole device goes offline, searching for the WAN signal ie No LAN connectivity, no DHCP No dns etc. - \> brain dead, but a known issue see here:

\> WAN/LAN auto-sensing - Home Network Community (tp-link.com) Many many posts about this issue.

This is exactly what I was afraid of!

I\>  had to buy a diff product as this has still not bee resolved. I have Deco X50's I hope to use them one day when there is a good firmware upgrade.

Could you please share what exactly did you buy? I'd prefer a mesh system myself and it needs to support over 100 clients, not like some older systems - only 64 and then they freeze and only a cold restart helps to recover it.