Why isn't my cellular data working?

I have an unlocked and jailbroken iPhone 4S. I know where the cellular data is at but when I turn it on it doesn't work, & it even says 3G on the corner but I don't know why it's not working! Can someone help? Thanks.

It is possible that you might have to adjust your APN settings on your iPhone in order to get data working.

APN stands for Access Point Name and it identifies the data access services associated with your iPhone or any smartphone. In other words, these settings allow you to send/receive picture messages to other mobile phones.

To ensure that your APN settings are correct, try the following:

1. Make sure that your iPhone is connected to the internet. Using WiFi is fine if data is not available.
2. Go to the link below, unlockit.co.nz, using the Safari browser on your iPhone to automatically download the desired APN related to the carrier you are using. COST: FREE