You have the following options.
1. Log on to the dashboard of your modem and check the data downloaded there. You have to refresh/renew it manually if you want to see how much you've downloaded. Call 211 for the web dashboard URL and username password
2. Install networx to your laptops, my data manager to your other devices. Find out how much data they've used and add them manually. (maths urgh)
3. Call 211 and keep on pressing until you get the automated response if you want to know your data usage. Might take some time memorizing the number or you can listen to each prompt