I'm probably late in replying to this thread, but I bought th esame card and now my pc restarts evry time I shut it down. I've "cured" it by removing the card for now. The Belkin e-mail support said I should try switching the other usb ports off. My local computer shop said I should reject the card (Amazon).
Don't know about your modem I'm afraid. The other socket is for a power supply - scant mention of it in the instructions - rather a peripheral mention of powering the card allows a bigger drain on the usb ports to power devices.
Hope of use.