Diagnosis
The TJ-610 has a weak Wi-Fi radio. It is struggling to maintain a stable connection to your router, especially if the signal is marginal.
How to Fix It
Check the Wi-Fi signal strength on the thermostat. Go to Menu > Wi-Fi > Signal. It must show at least 2 bars. Reposition your router or use a Wi-Fi extender to improve signal. Ensure your router uses a 2.4GHz network with WPA2 security.
If That Didn't Work
Your router may be using a channel with interference. Log into your router's admin page and change the 2.4GHz channel to 1, 6, or 11.
Heads Up
The TJ-610 cannot connect to 5GHz networks. It will not appear in your network list if you are only broadcasting 5GHz.