Can Apple watch be "setup for family member" with xfinity mobile?

My iPhone13 is with xfinity mobile. But after I set a watch (SE) "for family member", the Cellular Setup claims "The xfinity mobile account on this iPhone cannot be used to add a cellular plan to ###'s apple watch". And on the watch's setting->phone, it shows: "waiting for cellular plan activation". After hours of phone call with xfinity reps, nobody could give me an answer/solution.

Does anybody have similar problem with xfinity or have any insight on this?

Both the phone and the watch have been updated with the latest OS.

Thanks a lot!

Sharon