Help Galaxy S3 i747 issue, phone reads "Sim Card Not Detected"

[ErkaustNoAme]

I have a Galaxy S3 i747, it was stuck on stock rom 4.1.2 because the AT&T Software update wouldn't work. Anyway, every single rom that I have flashed, whether Kitkat or Lollipop or what, the phone always reads "No Sim Card". However, the confusing part is, when I go to call someone on my phone, it works perfectly fine. When I check network settings, it states that I'm connected to a T-Mobile network, but the phone itself says "No Service", and data doesn't work. I suspect the problem has to do with a modem update present that I haven't recieved, but I'm not sure how to go about fixing this, does anyone have any input on what I can do to fix this?

Thanks so much