@Amds 

You can't use the internal DNS server on for the WAN Port DNS.

You enter the Local DNS Server on the VLAN DNS section.  

Hi @Amds 

Thanks for posting in our business forum.

I got you that you want to use the DNS server for filter or other purposes. Yet you cannot use that like @KimcheeGUN said.

But if you insist use that, you can consider port forwarding port 53 for that DNS server. In the end, still strongly recommend you use a public one on the secondary. Your DNS server is not a root DNS server which means it's not fully capable of resolving all domains unless your hosted DNS server is updating with one of them.

If you open the port, the DNS server IP would be your WAN IP and each DNS query will be sent to your router and server. This could add up the load of the server/router. So, at your discretion.

Another fact that you should know is that your DNS query is first sent to the Computer DNS(LAN DNS), then to WAN DNS if the LAN DNS cannot resolve it.

LTT - What is the difference between the DNS setting in LAN and WAN on a router