Easiest thing to try first is to unplug the box for 10 seconds and plug back in for a restart.
When my remote suddenly won't work except for changing the volume, this has always solved the problem.
My bedroom TV will not respond to change channels. New batteries put in before coming to u
did you happen to notice the LED blink(s) on the remote when you pressed the up channel or down channel or Xfinity buttons? was it one blink either red or green or 2 blinks on each push?