VPN over DSL behind Router

[Locust2000]

We've got VPN setup at the office with a fixed IP address to it.
I can connect to it from one PC which uses an internal PCI ADSL modem.
I CAN'T connect for a PC at a different site which is behind a Dynalink RTA020 Router. It sits on verifyling username/password then eventually brings up Error 771 Computer Did Not Respond.
I don't have any firewall etc setup on the PC that can't connect and haven't setup any port forwarding etc as I don't really know where to start.
Was originally setup by someone else and I just copied the settings from the working client to the new client.

 

[substitute]

The Dynalink user is probably doomed - it is running NAT so VPN messages can't get out and back. What I can read about the RTA020 doesn't give me much confidence that it can be configured otherwise - like most of its ilk it is designed for simple NAT/ICS/DHCP/Firewall with the minimum of fuss

Essentially you need a device that is aware of VPN and able to pass the packets to a nominated PC as if the router wasn't there. One of my clients uses a Watchguard Firebox SOHO which does the job but needs a separate ADSL modem. There are probably similar devices around now that include the modem.