Under the "Usage" menu under Settings, there should be a subsection titled "Cellular Network Data" where it will show u the Sent and Received data. Add the 2 numbers and u have your usage. You can reset these numbers by pressing the button below it. The thing is that u have to reset it manually every month and when u reset it it will reset the other statistics as well such as Call Time.
If anyone knows an alternative solution, please do share.
Hope this helped a bit.