It depends on the model of the modem.
IPCop supports relatively few broadband devices over USB (I know, I use it too) and support can be temperamental to say the least.
If the modem has an ethernet connection option then for the love of god use that instead. When connecting over ethernet, the modem automatically configures itself and then connects exactly as a PC would do.
With my IPCop setup, I have a couple of netgear switches on the green side to distribute the connection between the PCs needing it, and on the red side is the NTL cable modem connected using ethernet.
"Configuring" the modem connection like that is simply a case of pluging the cable in to the modem and IPCop and walking off.