Need help with ISDN router config

I have an ELSA MicroLink ISDN 4U router which I've been using without a problem with my network for over a year. Now I've built a Linux (SuSE 8.1) firewall to protect my home network, but also to give me ftp access to files at home. To do this, I need to be able to connect to the Linux machine, but since it sits behind the router, it only has a network address allocated by the router. Under Windows, I could get round this problem by using the LANCAPI driver that comes with it, but I can't seem to find a Linux equivalent. Does anyone have any idea how I could overcome this problem, without having to buy new hardware? Any help would be greatly appreciated! :-) Many thanks, -John.