If installing just the modem does not work - other users claimed this worked with just a modem - you have to install a complete STOCK Android 4.2.2 ROM. Then find and use RegionLock Away to blow the SIM lock away.
Just to make this clear, you MUST be on a stock ROM, and it MUST be Android 4.2.2. Nothing else will work.
If, after doing all this you still can't unlock, you may have no option but to pay for an unlock code.