Neil
What type of connection are you using? You have typed "dialup" but I guess you mean always on.
If so you simple add the Router's IP that sits between your servers external NIC and the outside line.
If you have a router supplied by your line provider then they should have given you this information.
I've had a lot of playing around to get IPSEC to work.
Don't forget to leave the router empty when adding the local network as pointed out by Steve Bush and you also have to add both the internal network and external network IP's for both servers. as pointed out by Ryan Sutton.
I've managed to ping the remote server but thats about it, No Windows or Apple network, FTP or SSH. I hope you better luck. If you do manage to get it all working let me know what you did.
Rob