According to a post on the Apple support forums, user joxesCA writes, “Received my iPhone 12 Pro on Friday. Activated it on Saturday. Sunday I drove for 10 mns and when I arrive to my destination I saw no reception bars and No Service. And in the middle of the screen in a grey box: Your iPhone is not Activated. The only way to get the connection back was to toggle Airplane mode to ON then OFF. The iPhone had the cellular mode to 5G Auto. I switched it to LTE to test. And it was the same.”
They claim that they then tried to reset the network settings but found that it did not help and the issue kept happening again. It also does not seem to be carrier specific as users on AT&T and Verizon were reporting similar issues. According to speculation, it has been suggested that this could be due to the way the iPhone 12 is handling switching from one cell tower to another, which is why connections are dropping.
Apparently Verizon is aware of this but at this point in time, it is unclear if this is an issue that needs to be fixed by Apple, or if it might be a carrier issue, so we’ll just have to wait and see.