I have been having the exact same issue for about three days now. iPhone 7. Drops service out of nowhere, gives me an error message that I need to update carrier settings... no updates available. If I remove SIM card and put it back in, sometimes it will reconnect. Sometimes I can get it to reconnect by going in and out of airplane mode as well. It seems to have a mind of it's own and goes in and out when it pleases now.