Hi does the phone show any signal or anything? Only final option which I understand you may not want to do it a factory reset. I know its a big thing and make sure you do a back up. If you decide not to do this or it does not work it would be a matter for samsung to sort. If I can think of anything else will write another reply on here
Your sim card tray could also be faulty or dirty you could also try gentle blowing into the sim tray. Also if you have access to a spare sim card see if that works in the phone. It is mostly likely a major software bug thats occurred or your sim card reader has gone wrong. What network are you on? You could also request an esim see if that works
It says emergency calls only. I'm on three. They also did a diagnostic test and it's definitely my phone. It's not all the time, I used Google maps to get somewhere and it was fine, after that it dropped again and I couldn't use it going home 🙈 Never heard of an esim?
Esim means the phone cam download a sim and it stores it for you until you get rid of the phone it's mint. Not sure if three does it though. Sounds like the phone is going to have to go into samsung. You could try the factory reset up to you.
It DOES sound like a phone fault TBH, especially if you tried manually entering the network settings, and it shows no phone signal. Have you tried switching on Wi-Fi Calling? I leave mine in all the time. When on, there is a handset icon at top next to phone signal.