For the Linksys router you'll need to browse it. I don't seem to be able to find directions for wrt54gs but I'm hoping one of the following models will be identical: 1.
Port Forwarding for the Linksys WRT54G OR perhaps 2.
Port Forwarding for the Linksys WRT54G v1.1, v2 & CA And you'll also need to Force IP within LW.
LW settings instructions for Force IP (& sample image) Tell us how you go. And do another Bug Report & see if it's True/False.
(I think your modem should be safe!)