You’d have to be on the outside network edge of the router if you don’t trust the router, so if it’s ALSO your modem (ADSL/Fiber/Cable), then it’s a lot harder as a problem.
If you want a cheap enough way to do it otherwise, just grab a network switch that supports monitor/capture ports. Have a VLAN that just passes traffic between your modem & router, and also copies all traffic off the VLAN onto a monitor port that you can dump for analysis.
I have a little NetGear GS108Ev2 that I’ve used in the field for this explicit purpose, the only downside was having to use a Windows Vista VM to configure it in the first place. No VLAN, just dump all of ports 1-4 to port 8. Ports 5-7 pass traffic but don’t dump.