Resume function is not working within Xfinity for Apple TV streaming

Reset how?

Try signing out of the app. Then sign in again. That has reportedly worked for some users with similar issue.

Try a cold restart of the media box (not standby-wake, but a real restart).

Else contact the Apple Support staff for further assistance. Contact Apple for support and service (phone numbers) or start your support request online (chat/​email/​callback).

Have you verified that the Use Play History setting is still set to ‘on’?