How to get 3G on my phone after signing up for a plan?

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Hello there,
My dilemma:
I bought a new Tata Docomo SIM card and I signed up for one of the 3G plans. It doesn't seem to be working. Could someone walk me through the steps on how I can get internet working on my phone?

Things I have tried already:
I messaged "ACT 3G" to 53333 and got a reply that said my 3G was already active. (but still no internet!)
I tried messaging "INTERNET" to 52270 for settings and it always returns a message saying: "The request cannot be processed; try again in one hour."

Other Information:
I have used Tata Docomo 3G with a different SIM card on the same device (Nokia Lumia 720) before and it worked like a charm. My internet settings on the 720 are as they should be to use 3G, so I'm pretty sure that's not where the problem lies.

Any help would be greatly appreciated.Cheers.
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