ATT Prepaid Mifi

I bought a prepaid mifi for my mother as she is in a rural setting and doesn't have many online options. I am new to prepaid data services so I am learning this as I go. Anyway, I knew there would be an adjusting period to how long the data would last using basic internet surfing. Needless to say she went through the initial amount pretty quickly. I since purchased more data (twice now) but it only stored the cash amounts in my account and does not give me any data credit. What am I missing? It looked pretty easy, you buy it-run out-buy more... But, obviously, I haven't learned the nuances of this service. TIA