DIAGNOSIS The thermostat is losing its network configuration, often due to a weak Wi-Fi signal or interference from the HVAC metal wall plate.
FIX Check the signal strength in Settings > Technical Info > Network. A value below -65 dBm is poor. Restart your router. On the Nest, go to Settings > Reset > Restart (not factory reset).
IF THAT DIDN'T WORK The 5GHz band can cause instability. Force the Nest to use 2.4GHz. Temporarily disable the 5GHz band on your router, set up the Nest, then re-enable 5GHz. Alternatively, create a separate 2.4GHz-only SSID.
HEADS UP If your router uses WPA3 security, try switching to WPA2/AES. Early Nest software had compatibility issues with certain WPA3 implementations.