Nest Thermostat will not stay connected to Wi-Fi

The checks in more detail

After a router or password change, the thermostat must be told the new details in Google Home, under the thermostat's Settings, Wi-Fi, or it needs a reset and set-up. Routers that merge their bands under one name can confuse it, so try separate names. Restart the router and the thermostat from the Reset menu. If the thermostat shows Offline in the app but the display works, the Wi-Fi link is down. A thermostat that has a power problem also drops Wi-Fi, because the radio is the first thing to go when power is short, so check for a low battery message and fix the wiring before chasing Wi-Fi.

Good Home Automation's guide notes that weak batteries cause Wi-Fi disconnections. Hidden networks and networks with captive portals do not work, and a very long password with special characters sometimes causes trouble. If the thermostat joins and then drops at certain times, the router may restart at night or channels may be crowded. Setting the router's 2.4 GHz channel to 1, 6 or 11 helps. Moving the router a few meters or adding a node at the thermostat's end of the house is the most effective fix, because signal at that spot is the cause in most cases.

If the thermostat is in a hallway or on an outside wall, signal can also be weaker through external brick, and a mesh node on the same wall's inside helps. Smart plugs and other 2.4 GHz devices share the same band, so if many devices are on that band, the thermostat's connection may be crowded; moving some devices to 5 GHz frees capacity. Record when the thermostat goes offline to see whether it matches heavy use.