Thanks, but unfortunately, you did not mention a Time Capsule at all in your first post, so that changes things. Additional clarification on some details will be needed.
Sorry for the questions, but we really don't know any more about your setup than you can tell us.
We understand from the new additional information that you have a Time Capsule that is working correctly now, and that it is providing a wireless network signal at this time.
Will the AirPort Express connect to the Time Capsule wireless network, or will the AirPort Express connect to the wireless network provided by the new Xfinity modem/router?
Or, does the AirPort Express connect to either the Time Capsule or Xfinity modem using a wired Ethernet connection?