Adding Second Domain

Adding Second Domain

We have 2 separate domains in 2 physical locations. We connect through a site to site vpn. When I try to add the second domain It will not connect. I was told it is probably a network issue. They want to know what ports need to be opened between my PMP server and the Domain controller in the second domain, to be able to see Active Directory.
What ports and port types need to be opened for this to happen?