Having the same exact problem with my son's iPhone 7. He upgraded to 14.01. Got the cellular error. I tried to upgrade to 14.1 - upgrade failed. I tried to restore from backup - phone went into cycle between Apple logo and black. I forced phone into recovery mode and restored to factory settings. This worked but still have the cellular error, therefore can't even activate the phone now. I found this URL - https://support.apple.com/iphone-7-no-service/other-countries Going to call Apple today and see what they say.

At the poster below,  Bug_reporter - dude I couldn't even get into the that menu when I got the error.