That's not what he's asking for.
DNS running on the second pfSense interface (OPT1) doesn't matter because it will only do anything if the hosts on that subnet have it configured as their DNS server. Just don't do that. You could, in fact, set your hosts to use the DCs for DNS then have the DCs forward to pfSense for actual Internet DNS resolution.
When you create an OPT1 interface a DHCP server is not created automatically so there will be no DHCP server so no problem. If you want to just use LAN that was automatically created, got to Services > DHCP Server and disable it. Configure your DC DHCP server to give your AD clients the addresses of your DCs for DNS.
Neither has anything to do with whether you have internet access or not, other than without good DHCP and DNS, which should be provided by the DCs, the internet won't work.