My phone keeps losing service

I have a Samsung S21 Ultra. Over the past week or so I keep losing service. When I am connected to wifi I am able to make and receive calls as well as texts. However, when I am not on wifi my phone randomly loses service no matter the area I am in, even in areas that previously provided a strong and consistent signal. When I have no service, if my number is dialed it will not even go to voicemail. Turning on airplane mode then turning it off works to restore service sometimes but this is impractical to have to do all of the time.

I went to an xfinity store where they swapped out the sim card multiple times as well as reset all the network settings... nothing worked. They advised me to contact Samsung but I don't really want to be without a phone for what would be at least a couple of weeks. Plus I see that this has happened to others before. So before I go sending my phone away I thought I would reach out to see if this is on xfinity's end.

Now I keep getting messages from various accounts I have (Facebook, credit cards, bank, etc.) that say they will stop sending me text notifications or have deactivated my phone number in my account because they detected a network carrier change. Has there been some change in my account or with xfinity in general that is causing this issue? Can something be reset on xfinity's end to resolve this? Is this a bad update? What can be done? Help!