I can't send or receive MMS unless I unroot :(

So I know this is a weird issue. I've done my research and tested way too many times before posting this up :cool::cool::cool:

OK, so the problem is that I can't send or receive MMS on my LTE network. Also, I believe this causes me to have problems with my GPS as it sometimes it can be detected and sometimes it won't.

The steps I've taken to rectify the problem:

1. Completely unrooted phone and flashed stock firmware from sammobile.com.
2. Checked if MMS and maps were working on the stock firmware and they were. I checked the USSD code using *#1234# in the dialer and it was working.
3. Done the rooting process and flashed new recovery and new ROM.
4. Tried to send MMS and issue persisted again. Also, when I send the MMS it disconnects my LTE and stays disconnected till I reconnect again.
5. I checked the USSD again and it says "Connection problem or invalid MMI code"
6. I have flashed so many different modems and bootloaders and the problem is still there. Unless I unroot and stick to my network provided ROM.
7. I checked APN settings, manually set new ones up using the network APN settings and still nothing.

What is the problem here? Is it my phone? A hardware issue? WHAT THE.. :confused::confused::confused:

Any help is appreciated :fingers-crossed::fingers-crossed::fingers-crossed: