Ah, so you don't have like cable or DSL.
If standard ad-hoc methods are not working then it may be impossible, as that would be what I would recommend. It is a bit expensive, but they do have 3G routers, for example
Netgear MBR624GU. D-Link and Linksys also have them. In the long run, setting up a formal network instead of the ad-hoc one you are messing with may be safer and less aggravating, but I understand the need to save money.