Pc companion didn't worked. All was fine except that it says that "there wasn't any update for my phone". The same happened with sony exclusive "update service".
What worked was flashtools and a stock kernel/ROM available here
Once i flashed with this stock kernel/firmware, the "SIM network unlock PIN" became automatically displayed when i turn on the phone with a SIM from a different/locked network carrier.
Then as i already had the PIN (which i get from my old network carrier), put it there and it worked as it should. Unlocked...
No need to re-lock the bootloader or anything else, just flashing with stock kernel/firmware suffice.
Pd: I wonder why is that the custom cyanogenmod rom that i previously had, or for that matter, any other custom ROM, don't have this "SIM network unlock PIN" available.
It is almost insane to have a perfectly working and personalized custom ROM, and have to flash back to stock just to "network-unlock" the phone with a legitimate obtained code.