IPhone 4 won't connect to iMessage or Internet outside of Wi-Fi?

I have an AT&T iPhone 4 that I unlocked to use for CellOne. Whenever I try to use iMessage it says trying to activate… Or something like that. Whenever I go outside of my Wi-Fi I can't connect to the Internet even though I have to data plan so what's wrong?

Your internet is turned off. Go to a genius at the apple store and they can help you

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