Question [Q] Phone locked after flashing

Hello.

Here's what happened in short form

Model: i9300
Android Stock 4.1.x

1. Sim-Unlocked it via "hidden menu"
2. Used new carrier without issues
3. Switched back to original carrier (after a few months)
4. Flashed ROMs to hell and back. 4.2, 4.3 & 4.4.
5. Change to a new carrier (after about a year)
6. Phone is SIM locked.
7. Check with original carrier, they say "NOPE; you wont unlock it with us because of reasons"
8. Take phone downtown where shady people do magic
9. They flashed the "leaked" 4.2.2 and phone is UNLOCKED
10. I ask if I will be able to flash newer ROMs and they say "nope. Stick with this one"
11. Did nandroid backup
12. Flashed Neatrom android v4.3: WORKS
13. Flashed any non-stock ROM: No network
14. Went back to neatrom: works again
15. Used Archi's Archiport: WORKS
16. Flash any non stock ROM over it, whether 4.3 or 4.4: No network

So, im not sure what did the people downtown did. But everytime I want to flash a non stock ROM, it wont work.I tried making a backup of EFS while in STOCK, then flash non stock ROM, and restore the backup, but no luck.

Any idea?