Ideally, based on this comment, you probably would want to have a setup like this:
Iperf client ---> lan ---> router (Archer A7) ---> lan* ---> Iperf server
(* the lan cable between router and iperf server should be plugged into the wan socket of the router)
That is, because routers are optimized to route traffic, instead of simulating and calculating Iperf measurements. Running an Iperf server on your router uses the CPU, which might have an impact on the measurements.
But, as a rough first guess, you can also run it on the router, like this:
Iperf client --- lan --- Router (Iperf Server)