The unlock is no longer possible after the August update without downgrading. There's a workaround though:
Since your device is already unlocked, you can root it and then write the old abl and xbl from the stock B2N firmware.
Once you downgrade the abl and xbl, then you can go back to download mode and unlock critical using the same unlock key.
After that you can flash the device using OST LA to revert to full stock ROM.