AirPort Extreme disconnecting
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.