DIAGNOSIS: Intermittent Wi-Fi dropouts are causing the connection to fail. The thermostat reconnects quickly, making it appear fine on the wall.
FIX: Improve Wi-Fi signal strength. The Sensi ST55 has a relatively weak radio. Use a Wi-Fi analyzer app to check signal strength at the thermostat location. A reading below -70 dBm will cause dropouts. Consider a Wi-Fi extender.
IF THAT DIDN'T WORK: Router settings may be too aggressive. Extend the DHCP lease time for the thermostat's IP address to 7 days or more. Disable any "Wi-Fi optimization" or "smart connect" features that may cycle bands and disconnect the thermostat.
HEADS UP: These alerts are triggered after 4 consecutive failed communication attempts. You can disable them in the Sensi app under Thermostat Settings > Alerts & Reminders, but this masks the underlying connectivity problem.