Saving Traffic Analyzer data

[Larry Johnson]

Asus RT-AC68U. I use a satellite internet service and need to monitor my usage because of data caps. I installed a USB stick on my router and thought I was in business as my traffic analyzer data seemed to be saved. Before the USB any reset or power interruption lost all the data.
Some event of which I am unaware caused all the data to be lost again.
Can anyone provide instruction on how to use the USB to save data.
Thanks