Data Uage Mointoring???

So i recently switched my ISP and now have data caps. Come to find out, that the family is using a huge amount of data. I've tried looking into someway of monitoring usage by device and everything i found said to use DDWRT. I installed the newest kong version and I dont see where this is an option to view. Trying to figure out which devices are using the most data. Any info would be much appreciated.