Ping fails via RAS but works via LAN

Problem is with dialling in (connecting through NT4 Server) to connect to our Terminal Server box so users can run a session using the new accounts software: it worked initially, but subsequently failed to find the Terminal Server machine or find the host name. So I tried ping which leads me to think its a network problem not a Citrix/Terminal Server one.

When I establish a dial-up connection to the NT4 server and then ping this server with its ip address, it times out; pinging any server byname comes back with host unknown.
The fact that everything works across the LAN suggests its a RAS -related issue.
Any ideas gratefully accepted!