Not had experience of that model, but an older one with attached USB 3g dongle.
Draytek's website www.draytek.co.uk
is very helpful, but essentially I have found the following to be of use in the Draytek web interface.
Online Status - My connection uses WAN2 and I can see the TX and RX packets for the current session - though this is not useful if your connection drops and reconnects a lot.
Diagnostics - Data Flow Monitor - If you enable this is will show each individual connection to the Draytek from your attached devices in real time - useful to check if any of your PC's are sending/receiving more than you think!
This next I have not tried - Port Mirroring - sets up a duplicate Ethernet port to track data from the main port and log it. Possibly useful for you in this situation - the web site gives some information on setting it up.