Unhappy Support Request - Network Issues

Hello,

I recently purchased an i9305T 2nd hand but I am continually getting network errors with my Vaya (Optus) Sim.

I have not been able to get mobile data to work at all, using various configurations.

I have tried flashing a custom rom (CM Nightlies) and flashed various modem packages but still the same problem. I can send & receive texts and calls for a few mins, but then it comes up with network unavailable and nothing will work.

I'm not sure if the phone is locked, and at one point I figured it was locked so I purchased an unlock code but I can't use it as I am unable to use ANY USSD codes (I have tried heaps) except for the one to check my IMEI (Which is fine).

PLEASE HELP ME!!

Thanks,