Samsung TV and Xfinity Xi6-A box

I've seen posts over the last three years about how Samsung TVs are not connecting to Xfinity.  Some blame Comcast, others point to WiFI protocols or security protocols on the internet.  I would hope that Comcast has solved this problem by now, or at least can tell us what to do!  
I have an older model Samsung smart TV (series 7, QN55). When I got it, I connected my old Comcast box and everything worked fine.  About 3 months ago I started getting signal drops (sudden loss of programming).  I have an expensive internet service with Comcast.  However, my connection is still cable, not optical in this area. So, I thought maybe I should exchange my old box for a new one that might be more compatible...  Well, it basically just doesn't talk to my TV.  It is recognized (not automatically), but there is no picture.  The TV itself is fine, with streaming services and all.  But I want the local sports (baseball) and some other streaming that I can't get without Xfinity. 
One big difference between the old box and new is that the old one connected directly to my coaxial Comcast cable; the new one is only for WiFI.  My WiFi  system is not from Xfinity, if that matters. WiFi signal is strong near my TV and box.  I've tried everything I can think of, including plugging in the coax cable to the Antenna of my Samsung.  No luck.

Should I just go back and get an old coax box from Comcast?   A new TV?    A different brand?