Windows Server 2000 remote connectivity problems

Hi:

A client in south Florida is running a Server 2000 and SBS 2003 and they need to allow a 3rd party software company to access the client server to provide updates to their software. The domain is configured with a VPN between the clinic and a home office. Each node obviously has its own router.

The 3rd party software company was given an AD user profile which is on both the Server 2000 and the SBS 2003 with

1) domain admin
2) domain users
3) remote desktop user permissions.

I have provided the 3rd party software company with the router address for the server at the clinic (which is where their software is located) and they are unable to connect. What am I missing?

Thanks in adavnce for your help.

Best wishes and Happy New Year!!!

David