I recently ran into an issue where if my phone ran out of battery, when I recharged it it wouldn't pick up a cell signal. For reference, I have an iPhone 13 Pro with destroyed back glass and visible water damage on the back. My phone isn't in great shape but the screen is unscathed and the phone works mostly fine. Anyway, I was left without service for a week until I found this solution, so I'm posting this in case anyone else finds themselves mysteriously without cell service after recharging their phone. What you have to do is let the battery run out again. Once the phone is dead, remove the sim card and boot the phone. Once on the lock screen check if you get an SOS signal. If not, let the phone die and try again. If you do, put your SIM card back in and you should have normal cell service. This took me a few tries and the problem can resurface if you let your phone die, but this is the easy solution.