SGH i337M suddenly doesn't read SIM and has no popups.

So I recently sold my SGH i337m and the customer has been messaging me telling me that absolutely nothing happens when he inserts his SIM card. No popup saying no sim detected, nothing. When he goes to settings > more networks > mobile networks there's a pop-up that asks him to "insert sim card", and that's about it.

I know for a fact I sold him the phone WORKING as I made sure multiple times to test the phone with my SIM card and everything worked before I sold it to him.

I've been troubleshooting with him but unfortunately I have not come to a solution yet.

Things I have tried:

1. Remove battery & SIM then put back in


2. Reboot in safe mode


3. Wipe cache in recovery mode menu


4. Wipe data/factory reset in recovery mode menu


5. Insert another person's SIM (still no pop up, even when he uses a SIM from a locked company, it doesn't even ask for the unlock pin)

I'm hoping some of you guys have a solution because I've been looking all over the internet and no solutions have worked thus far.

Thanks in advance!