I just got a few B3000 from China, and originally was thinking to use as APs, so did not bother whether it doesn't enable VPN on CN version.
But later I realised that one of them could be used as a sub-router to host my VPN from work, so worked out to convert it to a Global Version.
root@GL-B3000:~# echo "US" |dd of=/dev/mtdblock8 bs=1 seek=136
root@GL-B3000:~# sync
root@GL-B3000:~# reboot
Reference: # Converting GL.iNet MT3000 (Beryl AX) from CN to Global