Diagnosis
The connection between the thermostat and the Vine cloud servers is broken, while its local operation is fine.
How to Fix It
First, reboot your home router. Wait 2 minutes. On the thermostat, go to Menu > Wi-Fi > Disconnect. Then reconnect to your network and re-enter the password. Finally, force close and reopen the Vine app on your phone.
If That Didn't Work
The thermostat may need a hard reboot. Remove it from the wall plate for 30 seconds, then reattach it. This forces a fresh connection sequence.
Heads Up
If Vine's servers are experiencing an outage, your device will show offline in the app regardless of your local setup. Check Vine's service status online.