Network unlock code not working

I have bought a used Motorola Defy (MB525), rooted, running CyanogenMod 9 (ICS 4.0.4). It is locked to work with only a specific carrier. I bought an unlock code, and unaware to this the previous owner got an unlock code from the carrier. Both are the same, so I suppose the code is correct. When I insert a GSM SIM card from another carrier and turn on the phone, I get a message that roughly translates to "insert SIM network unlock PIN" (see image below). When I insert the unlock code, it blinks a message VERY fast, I can't even read it, then it says "failed on requesting network unlock", or something like that. I have performed this more than 3 times, does it mean my phone is locked forever? I have also been told to remove CM9 and reinstall the stock ROM in order for the unlock to work, but (1) I have no idea if this would *actually* help, and (2) I have never changed a ROM before and, althouh I used CM9 to create a nandroid backup, I have no idea how to reinstall CM9 and then restore the backup.