I am having the same problem. My cellular data won't even show up and it keeps saying that it's an error. Going to try airplane mode overnight and try again.

Turning off my phone and then taking the sim card out and putting it back in has worked best for me. Longest I've had service is 24 hours

Finally got an appointment set up at Best Buy. APPLE HAS A RECALL ON PHONES EXPERIENCING THIS ISSUE UNTIL DECEMBER 18TH. I don't know why apple doesn't say that when you talk to support, but they don't. This is only until DECEMBER 18TH so get it done now if you still have that problem. I went to Best Buy to get it fixed. Apple wouldn't let them give me a loaner phone bc apple sucks. I called Apple support to get a loaner phone, but they're useless so I have to get another appointment in 10 days so my phone can be fixed. APPLE can't send you a goddamn phone so you either have to hope whereever you go has a backup or you're without a phone til they get yours