help with remote XR15

I have tried every possible code and every possible solution, but I am unable to have my XR15 remote adjust the volume on a recently-added Amazon Fire soundbar.  The Soundbar is connected to the HDMI ARC output, the Xfinity Cable is connected to HDMI 2.   The XFinity remote does channel change and other functions, but will not turn the TV on and off and cannot adjust the volume or mute.   Meanwhile, the Amazon Fire TV's remote controls on/off for the TV and adjusts volume on the soundbar.  So I can get everything accomplished with two remotes, but of course it will be helpful if the X15 could control the on/off and volume.   THanks very much