Why does my phone function as if it has no service when I have 2 bars of LTE?

My phone is an iPhone 12 Pro, so a bit older by many peoples cell phone standards but it functions fantastically and is well taken care of. No scratches, cracks, dings, etc. My service is Xfinity Mobile, which claims to use Verizon towers/network.

Earlier this year, I started doing DoorDash to make a little extra money and it’s going pretty well. One thing I’ve noticed though is if I ever get sent to an area where I am downgraded to LTE coverage, if my phone has 2 (or 1) bars of LTE service, everything internet/network related just stops working.

I noticed it at first when the DoorDash app kept timing out and I couldn’t complete deliveries, but then I started checking other functions and nothing that requires service works. Safari, text messaging, phone calls, etc.

Do you think the problem is my phone, or the carrier? Can I do anything about it besides get a new phone, which I don’t want to do?