Currently Samba AD DC doesn't allow to set custom port for internal DNS server and custom port for DNS forwarder. Because of this it is not possible on one interface to use several DNS servers that doesn't provide necessary flexibility while building different solutions.
The problem was discussed in mailing list:
So I suggest to add possibility to set custom port for Samba internal DNS server AND/OR to set custom port for DNS forwarder.
To show that this requirement is rather popular here is the link to serverfault question that was viewed 2k times https://serverfault.com/questions/666972/possible-to-make-samba4s-internal-dns-server-listen-on-non-standard-port
as this was pointed out in the discussions before, this will not be made customizable. Use a different IP address or even better a different machine if you need to set up a diferent DNS server. Do also not continue the discussion you started on the mailing list here, please.
As I said that is a wrong decision. When we need different services on the same network interface we use PORTS but NOT VIRTUAL IPs and NOT DIFFERENT MACHINES.
And please, don't try to make other keep silence suggesting such wrong and silly (yes, it is silly) suggestion.