If you are using a wireless router with ADSL broadband, it should be a combined "Router/ADSL Modem" which connects directly to the phone line and then either wirelessly or by ethernet cable to the PC(s).
You cannot use a USB ADSL modem in conjunction with a router and it is the router, not one of the PCs, that connects directly to the ISP. The networked computers then get their internet access over the LAN from the router.
It sounds as though you have a main PC connecting to the internet with the USB modem and are using the router as a simple "network switch" to connect the two PCs. That will let you run File/Printer sharing but will not let the second PC get online.
You could get online with the second PC by setting up "Internet Connection Sharing" in the Host PC but that totally defeats the purpose of having a router.