How can playing some Solitaire, looking at pictures of my Grand-daughters on Facebook and now and then use g-mail use 70% of my data? Microsoft (or you) have been doing random/monthly updates - are you/they using up my data?
Kathleen, even if you use another operating system, that as well as many apps or programs on your computer and other connected device can do automatic updates, send files to cloud storage, even do an online backup if you don't turn those off in the settings of each app or program.
time to do the Ethernet test :) go to your myexede dashboard refresh to see where the data is, leave your modem powered on, remove the Ethernet cable from the back of your modem, leave it off as long as possible(longer the better) then check the myexede dashboard from public wifi or a cellphone data plan to see if any of the data was used. if it has then the problem is on their end, if it has not used any data then it is on your end.