Good. The upgrade script /zyxel/sbin/fwupgrade_extract_verify.sh checks the nas hardware tag against some tag in the firmware. When it doesn't fit, it refuses to install the firmware. On a 'genuine' 210 the tag is D401, on your box it is DB01. So if you change the tag, I expect you to be able to install the 'normal' 210 firmware.
You can change the tag by executing
/zyxel/sbin/mrd_model -s D401
This stores the value in the u-boot environment.
After a reboot you should be able to install genuine firmware. I recommend you to not directly install the latest, as I don't know if the flash partition sizes are stable over all versions, but first 3.20, then 4.10, then 4.40, and finally 4.41. The firmware can be found here