Samsung TV not recognizing Xi6-T cable box

Two weeks ago I got a text from xfinity / Comcast asking if I wanted a 4k cable box.  Well, yeah! No brainer!  I received it just over a week ago.  I unplugged my Xi5-P cable box and set up my new Xi6-T cable box.  I'm using a Samsung CU7000 TV, with the cable box hooked up to HDMI 3.  Did the troubleshooting on the app. No luck.  I would get the "welcome" screen,  but then after a minute would get "no signal" from the TV.  After giving up, went back to my original box and took the new box to the store to exchange it.  Second new box I was able to get set and working.  Yay! Then the next day, it wouldn't connect to the TV and I'd get "no signal" from the TV.  After a few more hours of troubleshooting, no luck.  I was able to get through to tech support, they said to take the batteries out of the other remote.  That worked for a day.  Then stopped connecting and got the "no signal" error again.  Returned that box and got a third new box.  Same issue.  It worked for a day then would not be recognized by the TV, even after getting the "welcome" screen for a few moments- then back to a "no signal" message from the TV.   Technician came on site, switched my cable to the HDMI 1, wow!  That worked. (Funny thing, the Technician told me that the channels aren't actually in 4k, only 1080.  Only streaming services can give you 4K.)  Then after a day, back to "no signal".  After doing the troubleshooting, restarting, updating the TV software,  etc, etc, etc, no luck.  Put the old cable box back on, because it works.  I give up.  After all this and support that can't fix it, I'm ready to get rid of xfinity and only stream SamsungTV.  Apparently I wasted so much time trying to fix something that doesn't even give me a better picture quality.