DIAGNOSIS Weak or unstable Wi-Fi signal at the thermostat location. The ST75 has a less powerful radio than some routers.
FIX In the Sensi app, check the Wi-Fi signal strength for the thermostat. It needs at least two bars. Relocate your router or use a Wi-Fi extender. Avoid mounting the thermostat on an exterior wall with metal studs.
IF THAT DIDN'T WORK Your router's DHCP lease time may be too short. Log into your router and set the DHCP lease time to 8 hours or more. Also, disable 'AP isolation' or 'client isolation' on your router's Wi-Fi settings.
HEADS UP A dual-band router? Force the thermostat to connect to the 2.4 GHz network only. The Sensi Touch does not support 5 GHz bands.