If your router doesn't support traffic mirroring/monitoring you could do it this way:
Get a network a switch that supports traffic mirroring/monitoring or get a network hub and plug it in between the router and the rest of your network. If using a switch you'll need to configure it accordingly for traffic mirroring/monitoring. If using a hub, no configuration is needed.
Install Wireshark (or whatever you like) on the computer you want to use for this Wireshark analysis. Note that with a switch you'll need to configure it to mirror the traffic transiting the switch TO the switch port that your Wireshark computer is connected to. This is why it's easier to use a hub rather than a switch. A hub by it's very nature will flood all frames to all ports, so your Wireshark machine will see all the traffic transiting the hub.
Start your Wireshark capture. Analyze the captured traffic. Enjoy!
Note that it's pretty hard to find network hubs these days. I don't know of any network equipment vendor that still makes or sells them, but you can probably find on one eBay for a few dollars.