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Unfortunately the WAG54GS is a combined ADSL modem with router. That just will not work on cable broadband. Cable and ADSL are different connectivity technologies.
You need to take the thing back and acquire a wireless broadband router, the Linksys WRT54GS will do just fine.
The only difficulty then might be the cable modem you have. You say it is USB (the connection between modem and PC). Is there an ethernet port as an alternative. Unless there is an ethernet port you will not successfully connect the cable modem to the modem.