Hello yang2,
I apologize for the inconvenience you've experienced with your Nest Thermostat going offline. Let's jump in and get it back online!
Due to its reliance on its surroundings, interference can be challenging to locate and eliminate. To be sure, which Nest Thermostat do you have, and how far away is it from your router? We can try our best to diagnose and fix interference issues, but in the end, there could be a problem with the connection. Could you give a summary of every step you have attempted? Here is a troubleshooting guide for situations where the Nest Thermostat is offline.
Keep me posted.
Regards,
Mark
