I have had this same problem since we switched to Xfinity mobile a few weeks ago. Terrible connection and dropping calls all over the place - even when looking at towers I know are Verizon towers (which is what Xfinity mobile supposedly uses). When I do get decent connection, it goes up and down randomly when I'm in the same spot.
After hours on hold, was able to get through to tech support, who had me do the same network reset. That didn't help. They finally put me through to their "next level" support, who also couldn't help and had me contact Verizon (carriers can redirect the signal through a push function from their end - did this a few times over the years with AT&T after a tower would go down). When I finally got through to Verizon support, they had no idea why Xfinity sent me to them...and connected me back to Xfinity!
I'm starting to explore my options to switch again, because this is ridiculous. Might save a few bucks, but not enough to make this level of service worth it. If anyone has any suggestions, it would be greatly appreciated because this is just a hassle.