The manual onboarding requires the AP SN# with 14 or 16 chars BUT the AP SN# is only 10 chars long ?
I've checked another one of my 3912 which is connected to my lab V2110 and it looks like because "reasonX" 00000 is added to the SN# that is printed on the AP and box.
It would make sense to either print the correct SN# on the box/AP or change the Defender app to support the short SN# that is visible on the box/AP.
-Ron