to see all traffic you can use TCPdump. Tcpdump is command-line packets sniffer tool which is used to capture or filter TCP/IP packets that received or transferred over a network on a specific interface. You can simply go to Gateway WebUI System > Administration > Troubleshoot There at the Troubleshoot section, at the bottom you will have Enable TCP dump option, Enable it, choose mobile interface (since as I understand you want to get information on mobile data used) then Save & Apply the changes.
After doing that leave TRB to operate to accumulate the data, then go to same section of WebUI and download TCP dump file for investigation. Use WireShark application to analyze TCPdump file.
Or use CLI/SSH client and run tcpdump command to see traffic in real time. You may refer to this link (section 7.) for more information: