One way to find out, although cummbersome and very time consuming, is to pause all your devices and then turn only one on and monitor it. Then the next , and next, each day until you can isolate the particular data usage of each device.
FYI, if you are using a streaming device make sure you back totally out of that app. Just turning off your TV does NOT necessarially turn off that app. In-other-words, that app is still still active and using data!
... View more