No Cellular Signal/Unable to Activate

I was getting ready to sell me iPhone 11 Pro recently and I wiped it and updated it to the latest iOS. When a potential buyer tried to get activate the phone it kept getting hung up on the activation screen, eventually displaying an "Unable to Activate" message. I have brought it into Apple twice, both times they said you can get a replacement phone for whatever price. I've tried the hard reset/online internet reset with apple a few times, hasn't worked. Wifi and Bluetooth work fine. Anybody have any luck with a problem like this? I was thinking it might be the cellular antenna or chip, it doesn't come up with the "SOS" in the top right, just keeps searching.