If I'm understanding you, you have an NT 4.0 server acting as your router to the Internet. All IPs, both internal and external, are routable on the Internet. If that's not the case, please let me know how it is configured.
Check the IP configuration of one of the RAS clients after it connects. The default gateway should be the internal, routed interface of your NT 4.0 Server. The easiest way to do this is to have one of the RAS clients run the IPCONFIG /all command at a command prompt, and look for the line that shows "Default Gateway". If this isn't the internal, routed interface of your RAS server, change your DHCP server settings so that RAS clients receive the correct IP configuration. If it's already correct, make sure the Windows 2000 RAS client can ping it successfully.
If you can't ping the RAS server, you have a connectivity issue that is probably the result of the DHCP server handing out incorrect IP configurations. If you CAN ping the RAS server, try performing a TRACERT to a remote system on the Internet (past the RAS server). If TRACERT shows connectivity past your RAS server, your configuration is correct--but your ISP has a routing issue. If TRACERT doesn't work past the RAS server, and the default gateway is correctly configured, then you have a routing problem on your NT 4.0 RAS system.
A couple of years ago I wrote a book about routing and NT 4.0--it's now available online. It should have all the information you need:
This was first published in October 2001