DIAGNOSIS: The thermostat has lost its Wi-Fi connection. This is often triggered by a router reboot or a change in network credentials.
FIX: On the thermostat, press Menu > Settings > Wi-Fi > Reconnect. Select your network and enter the password exactly. The Wi-Fi icon will flash blue during connection, then turn solid blue.
IF THAT DIDN'T WORK: Your router may be blocking the connection. Ensure your 2.4 GHz network is enabled. In your router settings, temporarily disable AP isolation/client separation and any firewall rules blocking IoT devices.
HEADS UP: The Sensi ST55 cannot connect to 5 GHz networks. If your router broadcasts a combined SSID, you may need to split the bands for a reliable connection.