Red X in middle of dark screen

You could try swapping HDMI cables around and see if the problem goes away on the AT&T TV device or starts up with the Apple TV device. Replace the cable it that's the issue. Let us know.

@CGDMD 

Is your set-top box wired or wireless?

You're going to need to perform a non destructive factory reset by connecting the receiver directly to the gateway via Ethernet cable.  The procedure is done by unplugging the box three times spaced by a few seconds. On the final power up, you will see a gray gear and progress indicator. It will take some time.  Once done you can return the box to its proper home.  

My set-top box is wired.

@CGDMD 

Then go ahead and perform the procedure in place.   
I’ll also suggest putting your equipment on battery backup.  

One more thing.  If the wired connection back to the gateway is faulty, the procedure will fail again with the same results.  Check to make sure you see the green link LED on while the box is powering up on the third plug-in.  

I appreciate all these suggestions.  Unfortunately they did not fix the problem.  I called AT&T and after they also ran various tests and tries to fix, they determined that the cable box needs replacement.