DIAGNOSIS The thermostat is failing to establish a local connection with your mobile device during the provisioning process. This is typically a phone or network setting issue.
FIX On your phone, ensure Location Services/GPS is turned ON. Both iOS and Android require this for Wi-Fi setup. Disable any VPN or ad-blocking apps on your phone. Force close and reopen the Sensi app. Delete the thermostat from the app and restart the setup process from the beginning.
IF THAT DIDN'T WORK Your phone may be defaulting to a 5GHz Wi-Fi band. Temporarily disable the 5GHz band on your router, forcing the phone to use 2.4GHz during setup. Ensure your phone is connected to the 2.4GHz network you want the thermostat to use.
HEADS UP The thermostat creates its own temporary setup network. If you have 'Smart Network Switch' or 'Auto Network Switch' enabled on your phone, it may leave this network. Disable these features in your phone's Wi-Fi settings during setup.