I had the same problem (I think!) with my linksys wireless router - unfortunately only "B" speed. Having finally managed to get through to the linksys helpline, I stumbled on a guy who seemed to know what he was talking about.
What I had to do was to clone the MAC address of the network card that was originally set up to access the network directly. Apparently ISP's use MAC addresses to stop people sharing broadband connections. By putting this address into the router, the ISP assumes that there's only the original machine connected to the network. Needless to say, the ISP doesn't tell you this.
If you think that this may be useful to you, PM me with your e-mail and I can send you the "how to clone the MAC address crib sheet."
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