Not sure if you've found the definitive answer to your question, but if you have the time and money and your modem and router are separate, you could purchase a small 5-port managed switch (i.e. Netgear GS105Ev2) that has port mirroring and install it between the modem and router WAN port. Configure the switch to mirror the port connected to the router WAN port to one of the remaining available ports on the switch. Connect an ethernet cable from that port to an ethernet port running in "promiscuous mode" on a PC or Laptop. Install network sniffing/analyzing software on the PC or Laptop such as Wireshark (free) to capture and analyze the packets. Good luck.
