The WRT54GS is - if it is a derivative of the WRT54G - a cable/DSL router, and is therefore totally unsuitable for an ADSL service. It has no modem component and is designed to work with a cable modem. You require an ADSL modem/router which you must use without your existing modem. The latter should be removed from your PC and its software/drivers must be uninstalled.
You can use a router entirely wirelessly, BUT: you must use an Ethernet cable when performing initial setup via a client PC's browser and whenever you adjust a setting that affects the wireless link (including security settings and ADSL connection settings). Therefore, you must bear this need in mind when positioning your router if you do not have a laptop. A laptop would allow configuration to be performed at any time regardless of the router's placement.
By theway; router placement should be carefully considered to avoid interference.