A Hard One!
Way back when I first went Broadband, I spent a deal of time trying to get a USB Broadband modem working in Linux and eventually gave it up as a waste of time. I suspect the problem lies with the design of USB modems intended to work in a windoze platform, using windoze-provided resources, which are not easily delivered by Linux.
I got rid of the USB Broadband modem, and got a proper ADSL router, and found the performance was better and the setup very simple, not to mention the fact that I could run numerous systems off the same ADSL connection. I recommend you do likewise!