AirPort Extreme disconnecting

My AirPort Extreme routers started to disconnect multiple times a day, and all at the same time. Office2 and TV room stays green most of the time, but Daniel, Kitchen and Master Bedroom always disconnect. Once restarted - they always work, until they disconnect some number of hours later. This is a new problem. My network had been rock solid for years.

That could be the problem?

Over the past couple of weeks I have tried a number of things to fix my routers. I turned off IPv6 sharing, file sharing, renamed 2.4ghz and 5ghz networks to be on the same SSID (they were different before). Over time fewer routers were going down, until only 1 was a trouble spot. I reset that router to factory defaults and re-configured from scratch, but that did not hep.

At the same time, I kept getting email notifications from my thermostat’s cloud service, that it had been losing connection to the internet. I unplugged the thermostat from its power and after I did that, there were no more router problems.

I still don’t know the exact root cause. It has been 2 days of solid network stability. The thermostat has been in my house for 6-7 years, but it is of questionable brand that no one has heard of (Motison). Perhaps, it was poisoning the network.