Connect a computer to the WRT54G and use a web browser to log in to it.
(Device default IP address 192.168.1.1. Default user name blank and
password "admin". ) Change the configuration of the WRT54G as follows:
Under Settings/Basic Settings, turn the DHCP server off. CLICK THE Save Settings BUTTON.
Settings/Advanced Routing, change the Operating Mode from Gateway to
Router. CLICK THE Save Settings BUTTON.
Plug the ethernet cable from/to the upstream router into one of the numbered jacks, NOT the Internet jack. Change the IP of the WRT54G to a IP within your LAN's IP range. Reboot everything.