DIAGNOSIS It's a weak or unstable connection between the thermostat and your router. The thermostat's Wi-Fi chip isn't as powerful as a phone's, and things like distance, walls, or your router's settings can knock it offline.
FIX 1) Reboot the thermostat. Pull it off the wall plate, wait 10 seconds, and snap it back on. 2) If that's temporary, you need to improve the signal. The best fix is to get a Wi-Fi extender. Don't put it near the thermostat—plug it in halfway between your router and the thermostat to boost the signal along the path.
IF THAT DIDN'T WORK Your router might be the culprit. If it's set to use only the faster 5GHz band, the thermostat can't see it (it only works on 2.4GHz). Log into your router settings and make sure your 2.4GHz network is enabled and broadcasting its own network name (SSID). Then reconnect the thermostat to that specific 2.4GHz network.
HEADS UP Avoid fancy router features like "band steering" or "smart connect" that try to manage devices between bands. They often confuse simple devices like thermostats. Setting up a separate, dedicated 2.4GHz network for your smart home stuff is the most reliable fix.