Which thermostat do you have? There are two types compatable. If you search on ebay for xfinity home, you will see the newer model...if this the one you have to do you have This one:
https://customer.comcast.com/help-and-support/home-security/manage-xfinity-home-thermostat
https://customer.comcast.com/help-and-support/home-security/manage-xfinity-home-thermostat