Default Unlocked Samsung Galaxy S3 Won't Connect To Network

As the title states, I decided to fork out 20$ on an unlock code for my SGH-T999L. The unlock went smoothly, but I can't connect to any networks for some reason, whether I use my inactive AT&T SIM or my active MetroPCS SIM.

When I set my phone to auto-select the network, it tells me that my SIM doesn't support the network it picks. When I search for networks, I get T-Mobile, AT&T and "311660" on both SIM cards, which neither of them will connect to. I'm sure it has nothing to do with APN settings, as those are solely for MMS and Data if I recall correctly.

Any ideas?