DIAGNOSIS Weak or unstable 2.4 GHz WiFi signal at the thermostat location. This model does not support 5 GHz networks.
FIX Move your router closer or install a WiFi extender on the 2.4 GHz band. In the thermostat's WiFi setup, choose a 2.4 GHz network with a strong signal. Avoid special characters in your WiFi SSID or password.
IF THAT DIDN'T WORK Your router may be using a WiFi channel with interference. Log into your router's admin settings and change the 2.4 GHz channel to 1, 6, or 11. Also disable "band steering" if it's on.
HEADS UP A dual-band router with a single SSID for both 2.4 and 5 GHz often causes this. Create a separate, unique SSID name for just your 2.4 GHz network for the thermostat.