I bought an S20 ultra 5g this year which has this problem too. I purchased it 2nd hand and didn't realise the extent of the issue until the 30 day return was up. So its out of warranty, cant be returned and I have to pay for any repairs or checks. I never expected it to be such a useless phone considering how much it cost and how it is still a relatively new model. (I thought I was trading up!!). The issue is, it connects to a sim provider's network for a few moments then drops out displaying 'emergency calls only' or 'no service' and when it does connect it has problems registering on the providers networks. I work in London, surely you'd expect a decent signal there! I swap the sim cards (EE, GiffGaff, Vodaphone, O2) to my S10 and they always connect up straight away and work perfectly. I've tried every online fix many times, including multiple factory resets, no use. Checked mmc and mac settings, no problems, Sim card tray replacement, no use. The Samsung drop in repair centre tested the phone with software and said there was an issue with the motherboard. They quoted a replacement motherboard cost which is more expensive than the phone itself and dont guarantee that it'll fix the issue anyway. They offer a pittance trade-in and encouraged me to buy one of their newer models on display. The cheapest was £1300 - Hah! Buy another Samsung after this one?? I think not.
So I paid for the motherboard to be checked out by a Samsung motherboard expert I know, who said they are not easy to examine microscopically, because the circuit board is a type of folded over design in the later Samsung models. He could find no obvious circuit breaks or faults on the motherboard, but he fitted a new motherboard that he had for this model for me, just to see what happened. Well, within seconds, the new motherboard did exactly the same as the original one. It connected for like 15 seconds then dropped out. So we installed 2 new antennas just in case it could be those, then various new connections just to try and eradicate it as a hardware issue. That was no use either.
The phone is draining me. Did I just say 'phone'? It's not. It's just a very very expensive camera. The same issue exists for the sS21 and S22 from numerous threads I have read. Samsung are happy not fixing what they must know is an issue, either with a whole batch of cheaper modems/faulty motherboards/ bad software that they've used. Who knows??? Only Samsung it would seem. And they are not admitting it.
