A data bank can only be active when you have 30 Day, Long Term (180 and 360 Day Plans) and Mobile Broadband Plans. 

In order for you to use your data bank, you must first of all have available data in your data bank. Once your data allocated for your plan has been used up, you will automatically switch to your data bank. You do not have to go through any settings to change to data bank as it will switch automatically.