Mi3W SIM not recognised - peculiar case

Hi All. I have quite a strange case of my SIM card not being recognised by my Xiaomi Mi3W.
Here's what happed.

1) Purchased phone off friend with stock MIUI - all working
2) Inserted my SIM with mobile carrier A (still with stock MIUI) - all working
3) Flashed Cyanogenmod 12.1 Cancro on phone - all working
4) 3 days later, changed mobile carrier to carrier B - not working
Original SIM card from carrier A was in phone when my account with carrier A was erased.
Since my number was ported from carrier A to carrier B, the phone has had issues recognising SIM cards.
5) SIM card from carrier B not working, and neither are other SIM cards
6) I have checked with carrier B and my phone is definitely compatible with their network.
7) My SIM card is working on all other phones
8) The weird bit is that at the shop of carrier B, the shop assistant SIM worked, but a brand new SIM didn't.
9) I have tried reflashing Cyanogenmod. - not working
10) I have updated Cyanogenmod. - not working
11) SIM tray looks clear of dust/obstacles - still not working

Not to sure where to go to next. I guess I can try back to MIUI or stock android, but I'd love to hear what you think.