Wireless Solutions for the Neighborless Types

Hello,
have a quick question here. I have a client that lives outside of any metro area(nearest town being 15 miles away)they currently have a wireless router that is connected to a alltel wireless internet card via a usb cable(I know thats strange but it works better than other configurations)Because of their location the signal is really weak so the internet connection is weak at best, and they do have a range extender added as well. So my question is does anyone know of some type of hardware that we could add to make the signal stronger?
Thanks for your time.