DNS resolution not working over WAN

Hi all. Having a little problem that I need some assistance with.

I have a computer configured as an Active Directory domain controller, a DNS server, a router (via RRAS), and a DHCP server. DNS resolution through this computer (as in, a client computer sending DNS requests to the server) works fine on LAN, but when I try and use the DNS server over WAN, the connection times out. I did a port scan and found that port 53 is open, but the server will just not respond to DNS requests over the internet. How can I fix this problem?

Thanks,

Zane Kaminski