What have you tried? Have you seen this https://support.sonos.com/en-us/article/control-sonos-home-theater-with-your-tv-remote ?

In general, the Arc Ultra uses volume commands sent through CEC (HDMI-ARC) from the TV set itself, and isn’t connected directly to the remote. 
 

You probably need to connect the remote properly to the TV, and let the TV communicate volume to the Sonos Arc Ultra across the HDMI cable that connects it to your TV’s ARC port. 

Thank you Bruce 

Is the XFinity remote part of a settopbox? Because then teh TV should relay any commands it gets via CEC from the box to the soundbar.

Yes.

Thank you