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What's the speed of the connection provided by an ISP broadband, DSL, or dial-up have to do with any network setup wired or wireless? It has nothing to do with it.
This is what you need to do.
1) get a standalone modem that's running Rj45 as the connection type not USB.2) When you replace the modem, you have to tell the ISP about the MAC of the new modem, because such equipment uses a unique MAC and the ISP must know about this MAC so that the ISP can link/provision the MAC to your account so they can find your modem on the Internet.
3) get a standalone wireless DSL router so that you can plug the modem into the router that uses RJ45 as its main connection.4) The router and the wireless NIC should be from the same manufacture -- less trouble for you on the setup.
That's it and child can do it, but if you're afraid to do it, then have someone come in a do it and pay the $50 and be done with it.
Duane :)