HTC vivid will connect to wifi but not mobile data?

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http://androidforums.com – Hello! So I just got my hands on a used HTC vivid. I'm very pleased with how elegant it is and how smoothly it runs. I first connected it onto a wifi network where it performs quite well. The problem is I can't connect to the data network. I have a data plan from my Sim card that I transfered out of my LG thrive but the phone doesn't connect to the data network. It connects to the wifi just fine though. Anyone got any suggestions? Please ask me for info that you'll need, I have no clue what to provide you with. Thanks! P.S. On AT&T (General)