AD Sub-domain Joining - workaround?

Like most in a well setup AD site, I have only a subdomain for AD

WD EX2 Ultra has a limitation to only use root FQDN domain, since my forest FQDN is an internal sub-domain, this means I cannot join the device to AD and use the AD sec groups for ACL and drive mapping purposes.

Does anyone know a fix for this either in DNS, a hack on the device, joining via SSH, any other way. Seems crazy to offer a NAS and not allow to join AD sub-domains which is best practise.

I have internal DNS servers, any way using those to trick the device to finding the domain and joining it?

and I certainly do not want to create whole NEW forest and link back to the FQDN for one NAS, that would be crazy!