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When you disconnect your existing router, and instead connect the ethernet cable from the modem into the WAN port of the WRT54GS you must power the modem off for 30 seconds or so, then power it on and let all the lights settle down again, before you then power up your new router, and then any PCs connected to the router.
It's easiest to start off by connecting wired a computer to the WRT54GS, to ensure you have internet and associated connections, then later go wireless.