DIAGNOSIS Weak or unstable Wi-Fi signal at the thermostat location. The T6 Pro has a relatively weak radio and needs a strong 2.4 GHz signal.
FIX Move your Wi-Fi router closer to the thermostat, or install a Wi-Fi extender. Ensure your 2.4 GHz network band is active and not overly congested. During thermostat setup, stand next to it with your phone to ensure a strong phone signal, as the app facilitates the connection.
IF THAT DIDN'T WORK Perform a hard reset on the thermostat's Wi-Fi module. Go to Settings > Wi-Fi > Advanced > Reset Wi-Fi. Reconfigure the connection from scratch. This clears any corrupted network profiles.
HEADS UP The thermostat cannot connect to 5 GHz networks. It only works on 2.4 GHz. If your router combines bands under one name (band steering), temporarily disable 5 GHz or create a separate 2.4 GHz SSID for the thermostat.