Coax outlet in one room has no signal. How to diagnose/fix?

I have two coax outlets in my condo. One works fine and has active Xfinity signal. The other has no signal at all. I checked behind both wall plates and each has a single home-run cable going into the wall with no splitters. So they must connect somewhere further upstream that I can't easily access. Is there a way to test and diagnose a dead coax outlet without access to wherever the lines terminate? And any tips for getting Xfinity to actually dispatch a tech rather than routing through their automated troubleshooter?