Domain controller and client separated by linksys router, how can they talk to each other?

I would like to be able to log in to my server. The problem is my domain controller has recently been moved behind a linksys router (a WRT54G, to be exact).

The server I want to access has a publicly addressable IP address. The domain controller has a private IP address. What do I need to do to get this to work. If I move the domain controller to a public IP, I would still have machines with private numbers that aren’t able to see it.