Just wondering should the ISO image for a volume product key come from the VLSC?
Yes, ideally that is correct. Assuming you used evaluation media then as a work around , might also try using a kms key as intermediary step.
Dism /online /Set-Edition:ServerStandard /ProductKey:N69G4-B89J2-4G8F4-WWYCC-J464C /AcceptEula
then if successful use your own product key below after reboot
slmgr /ipk xxxxx-xxxxx-xxxxx-xxxxx-xxxxx
slmgr /ato
--please don't forget to upvote
and Accept as answer
if the reply is helpful--