I posted here a couple of weeks ago when a thermostat would no longer
connect to WiFi.
Two Nest Learning Thermostats (3rd Gen), upstairs and downstairs.
Upstairs Thermostat has been offline for several days. It can’t see the
network at all. Have reset the thermostat, factory reset, moved it next
to the router, even brought it downstair...
Thanks Mel.I spike to Google support yesterday. It seems it’s most
likely a WiFi chip problem, as @CoolingWizard suggested. They
acknowledged this is a know issue and will likely mean needing a
replacement unit (under warranty).