Post-Installation Wizard: primary name server not resolvable

Hello Virtualmin users

The post-installation wizard of virtualmin tells me, that my primary name server cannot be resolved from the internet [host not found: 3(NXDOMAIN)].

With the help of this forum, i managed, that the fqdn (linux server of a non-public local network) is resolvable in the local network [nat-router as name server (dnsmasq)] on the command line of foo-host itself.

What I understand is, that of course couldn’t be resolved from the internet, because it doesn’t exist in the internet (non-public local network).
But what I don’t understand is if I have to specify a DynDNS name server or my nat-router as name server.
Or if I can choose the option ‘skip check for resolvability’

What I have in mind:
I am going to create some ip-based [eth0:x] virtual-apache-hosts for a school. Hosting non-public websites that must be reachable from the intranet and the internet via openvpn.
I had the idea that i can use vitualmin/webmin to host these websites and that I can use fqdn as [vhost 1] [vhost 2]

Additional info (no idea if necessary):
The domain ist registered by DynDNS Custom and activ.
So I use to find my nat-routers wan ip and port xxxxxx to find my openvpn server [nat-router]. Then I use to find the login site of webmin/virtualmin.
I can also use from the local network without openvpn. is managed by dnsmasq [nat-router].

Any hint will very appreciated!!

kind regards, go4vmin