Only getting 480p on HD DTA - XFinity

This question is about "Only getting 480p on HD DTA", with XFinity-Comcast television equipment. I just hooked up a new 24" 1080p TV to our HD DTA via HDMI and although it initially flashes 1080p up in a box in the corner of the screen, it downgrades to 480p a few seconds later and I get a shrunken picture on the TV screen.

How do I get the full 1080p picture?

This topic covered Only getting 480p on HD DTA, and XFinity - Comcast cable tv service.