I thought I'd pass this on as this is an issue that I've not seen mentioned and frankly a little bit surprised by (especially for the 4th generation thermostats.
When I rebooted my router a week ago, suddenly both my 3rd gen thermostats went offline. (I was experimenting with a router based VPN implementation. I spoke with Google support trying to understand why a VPN would affect the ability of the Nest app to see devices it was connected to for years. I didn't get an answer so I disabled the VPN and rebooted the router again.
I was unpleasantly surprised to see that my previously connected 3rd gen Nest thermostats still could not be found by the Nest app (although they could be connected to the wifi manually).
The thermostats worked that way for a few days, and while I thought about it my opinion was colored by a previous sudden failure of 3rd Gen to be able to connect to any network or bluetooth.
So I bit the bullet and replaced them with 4th gen assuming that these would also support the latest wifi protocols and because it would allow me to ditch the Nest app and use just Google Home which I had due to the doorbell camera.
Disabled my wifi firewall for the setup and everything connected as it should. Not immediately, but about an hour after putting the router firewall back in service, the thermostat closest to the router (about 7 ft) lost its wifi connection. Just as previously, the Google Home app could not find the network through thermostat (the 4th gen don't offer the option for a manual network setup - WTF?). The thermostat furthest away from the primary router - about 40 ft (but I have a mesh / multiple wifi routers so there are no weak signal/deadspots) still connected without any issues. WT????
To shorten a too long story, I as I checked various router settings, I switched the wifi protocol from WPA2/WPA3, to just WPA2. Voila, instant wifi access on the troublesome thermostat. From this, it appears that the wifi hardware in the Google thermostats is not state of the art/current to even not so new protocols like WPA3 - even with the brand new just released 4th gen thermostats.
Just something to consider if you have issues with setup or keeping a wifi connection after setup.
