Using Xfinity XR15 remote to control Apple TV HD?

Hi!  I am using the Xfinity XR15 remote to control our Samsung TV (which has Sonos connected for sound.) I am trying to not have to use a separate remote to use Apple TV HD.  I can use Xfinity remote to switch from the cable box to the Apple TV input, but once I'm using Apple TV, I can't seem to pair the XR15 remote with it so I can use that remote rather than the Apple TV remote. (It won't "learn".) Has anyone had any luck with this?  I tried changing the XR15's mode to IR from RF (which basically disables voice commands) but even then it wouldn't pair. If this is something that's not possible, I'll quit trying.  But wanted to make sure I wasn't missing anything. Thanks!