My new iPhone 5 can't connect to the Internet besides Wi-Fi?

I'm pretty smart with apple products and technology so I'm pretty stuck. I got an iPhone for Christmas and everything ran fine. I had LTE, processor was fine, Wi-Fi was working even before iOS 6.0.2, etc… Okay well since I was home almost all day I was on Wi-Fi and never used my data. Well anyways my sister works at AT&T and is the one that gave me the gift and I want to know is there anything I can do before she helps me. Oh I almost forgot! I have rebooted three times, powered off, and turned off cellular usage various times and turned it back on. So any tips or help?

Well, i have verizon so i'm not sure if its going to be the same, but when i first got the iphone for christmas, i had to dial (star) *611 then i set up my iphone from there with my verizon account, as i said, i'm not sure if this is the same with at&t so i hope this works for you.

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