vpn configuration in win nt40

I’m attempting to allow a few remote users to access their company server from home using internet connections through a VPN using Win2k Professional and win98. They would be logging into a Winnt 4.0 Server, which is behind a dlink router using a static IP.And my nat port 1723 and 47 is forwarded to that server.and this same system i using for mail server and its working fine.
when i tried to connect to vpn server in clients i getting error 721 computer not responding.can u expain wht isthe cause for this.