Not sure you can get a month by month report. As you know you can get the usage for the current month by logging into your cellular account in Settings > Cellular
You can also see how mucH cellular data you used since the last cellular reset at
Settings > General > Usage > Cellular Usage
This will give you the data sent and received since the last time it was reset. You could reset it at the beginning of your monthly cycle and check it periodically to determine your usage rate.