There are some routers with USB ports. It sounds like the salesman is suggesting that your USB modem be plugged into a USB port on the router and be used instead of a cable or DSL modem for internet access. Interesting concept.
I'd first check on your wireless plan for how you are charged for internet time. If you have got the wrong plan, it could cost you a lot of money for two computers and several cell phones.
If it does work, and I've got my doubts how well it would work, yes you need a new router.