Why isn't my GiffGaff PAYG Internet working in my IPhone 3GS?

Basically I got an Iphone 3GS and the i put my giffgaff sim in it.
I topped it up £15 and got the 400 mins and Unlimited texts and Internet package as i always do.
Now when i leave my house (disconnecting the Wi-Fi) my texts and Phone calls are working but my internet isn't connecting.
Is there a way to fix this?

I don't know much about these things but I presume it is because you are not connected to the Wi-Fi?

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