I see a number of threads relating to solid red lights on Deco units, but they have been closed, so I'm opening another in the hope I might get some explanation as to why my Deco m9 Plus, 3 unit system has become unusable.

Firstly, I purchased my system in December 2020 and it has worked flawlessly for 2 years and 8 months, even through power cuts, and never required any attention.
A couple of weeks ago, this changed with the occurrence of the dreaded solid red light and “No internet connection” error message and I am now unable to get any of the units working again after wasting hours of my time troubleshooting them.

I have a very particular configuration, but as I say, this is not new and it hasn’t changed since they were first installed, so the configuration CANNOT be blamed for the issues I’m having.

I have a BT Smart Hub, which I use purely as a modem.  It doesn’t have a modem only or bridging setting, so the DHCP and wifi have been turned off.  I understand there are double NAT’ing issues with this configuration, but these have never caused me a problem.

The BT Smart Hub has IP address 192.168.1.254.

The Main Deco M9 unit is plugged directly into the BT Smart Hub using an ethernet cable.

The WAN settings on this deco unit are:
Internet Connection Type: Static IP
IP Address: 192.168.1.253
Default Gateway & Primary DNS: 192.168.1.254

The LAN settings are:
LAN IP Address: 192.168.0.1
Start IP Address: 192.168.0.3
Finish IP Address: 192.168.0.254
Default Gateway: 192.168.0.1

All devices (wired and wireless) are given IP addresses by the Deco unit, not the BT Smart Hub.

The other two Deco units are positioned around the house and use a 1gb wired backhaul.

After a couple of weeks of the Deco units having to be rebooted due to solid red lights and no internet issues, yesterday morning we had a very short power cut (literally off, then immediately back on).  All devices in the house recovered, including the BT Smart Hub, which had a good internet connection (I know because I’ve connected PC’s directly to it).
However, no matter what I tried, rebooting, powering off, restarting the BT Smart Hub i.e. all the things I’ve done over the last couple of weeks since the issues started, nothing would prevent them all from eventually showing a solid red light.  I also tried factory resetting them and moving them around so a different unit was used as the Main Deco, plugged into the BT Smart Hub.  After the factory reset, the system appeared to be up and running again, but later in the day the same issue occurred; all three Deco’s showed a solid red light, with no internet connection and finally the deco app didn’t recognize them any more.

I have raised this with TP-Link support and I’m in the process of raising an RMA.  However, the RMA notice states:

“If the faulty unit is part of a KIT/Pack, TP-Link only replaces the single faulty unit/part”

How am I supposed to identify which of the 3 Deco units is faulty when they all behave in exactly the same way and none of them are working?

I would really like to understand what is causing these Deco units, that were once so resilient, to be now completely useless?  Is this a hardware issue, a firmware issue or a configuration issue?

The firmware was upgraded to 1.6.1 build 20230403 last week, after the last issue and they appeared to work again for a week until yesterday’s power cut.  The configuration has been unchanged for 2 years and 8 months.