So I've tried everything published here and elsewhere to get my Nest Thermostat from "Account disconnected" on the device and "offline" in Home app, although it connects to my wifi network without trouble. I replaced the 2nd generation learning thermostat that I owned with the current model Nest Thermostat device (not the Nest E). I migrated my Nest account to Home quite a while ago and all was well until a week ago when I needed to update my wifi password. I have a complicated setup (ie. old and new gear including Hello doorbell, Outdoor IQ cameras, floodlight/cam, battery doorbell, Home MAX, etc.) that I was able to get back online and visible except for this Nest Thermostat! any thoughts other than deleting my Home and loading all from scratch?
If you changed the WiFi password for your LAN, you need only go to the Nest Thermostat, enter settings, go to network and store the new password. The thermostat ID would already be registered in the App. it is just the WiFi password that is needed to access the LAN. As long as you do not delete the thermostat within the app, things should still work.
Ken, AC Cooling Wizard
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