I have to report that I had this problem too (found this thread in the 'already being discussed?' display).
This past weekend, I travelled from the Midwest to the West Coast where my husband was sent for work (low risk, since we're both vaccinated). I discovered quickly that when moved from my home location to the coast and then back to the West (Colorado) for a connecting flight, my phone would not acquire a cellular signal until restarted fully ("no service" was displayed).
Toggling Airplane Mode did not resolve the issue and no carrier settings update was available.
I know that some report that resetting network settings can resolve issues like this, but since it loses all saved credentials, I'd like to check whether there are any other known solutions for this problem. Still, if it happens again I'l do that.
I'm on T-Mobile and the phone was purchased outright from Apple, so there is no carrier lock. Mobile/cellular data roaming is turned on. iOS 14.6 is installed and I do not have a beta profile and have never installed a beta version on it. Unfortunately, I didn't toggle LTE only, but I did have a 5G signal when cellular data was working.
Interesting that the original poster here also has a 12 Pro. Could be a weird glitch with that model. Mine's a 512GB, for what it's worth.