Prepaid Data Plan?

I have been using an old iPhone 3g using a prepaid att sim card for almost 2 years now. I recenltly discovered a forum that allowed the iphone to use the prepaid data services. http://http://gigaom.com/apple/how-to-give-your-old-iphone-new-life-with-prepaid -data-and-minutes/ I tested this before upgrading my rate plan and it worked flawlessly. After upgrading my minute to minute plan to the $50 a month unlimited talk, text and data package safari can't connect to the server all of a sudden. Before It was working and charging large amounts of data usage charges to my account. Any suggestions? I already bought the package so I am up a creek without a paddle on this one as of right now.