I posted to this thread a couple days ago with the same problem. I chatted online with apple and they called me and escalated it up the ladder. I then talked to a very nice rep over 3 days and they looked all over my phone. I had done the reboot, factory reset, etc. The problem ended up being that I had a Norton(Symantec) security app on my phone that was not compatible with ISO 14. I removed the app and no more problems with the cellular data cutting out and not working. If you have a security app(or vpn) try removing it. Hope this helps someone.

HELP, my iPhone 11 has turned into a paperweight outside the house. After installing 14 and 14.1 I get no cell data. I have tried everything and nothing works. If I put it on airplane mode and off I can get data for about 2 minutes. Also my cell reception for calls is a lot worse and I drop people where I never dropped them before. This has to be a software glitch, please Apple fix it.