Sorry, but most of the time, you can't.
The wap network is comprised of wap pages, which are (generally) scaled-down pages suitable for use with a cell phone browser, rather than a full-featured browser that you'd find on most computers. To serve internet to a computer, you're going to need full-fledged internet on your account in the case of most providers.
I work for T-Mobile, and we set up phones all the time to function as modems for laptop and desktop computers (the phone doesn't care which one you're using). Many newer laptops will also accept a sim card directly, but the principle is the same; the computer is still using a fully-featured http-type browser, and that requires access beyond the wap network.