Why won't my iPhone data work?

I recently bought an iPhone off of eBay. It is an unlocked at&t phone, and I'm using it with Straight Talk's unlimited prepaid plan, as it is cheaper than regular contract plans. The only thing is, my data doesn't work unless I'm connected to a local Wi-Fi network. Can someone help me resolve this please?

Straight-Talk doesn't offer data to unlocked iPhones. In order to get a data connection, you need to be registered with a legal iPhone carrier, such as AT&T, Sprint, Verizon

Yes it works, don't listen to the other dumb ***!

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