Hey 
the use of iperf3 is very simple and does not require much study 
I will now go into the use via SSH - presumably the plug-in only takes care of the installation.
On the router
- connect to your router via SSH (e.g.
ssh root@192.168.177.1) - update the software database:
opkg update - install iperf3:
opkg install iperf3 - start iperf3 in server mode:
iperf3 -s
On any client, e.g. Windows:
- Download iperf3: iPerf - Download iPerf3 and original iPerf pre-compiled binaries
- Connect iperf3 to the server via command line:
.\iperf3.exe -c 192.168.177.1
The IP naturally corresponds to the IP of the router. - Read the results.
Both programs can be interchanged - i.e. you can start the server mode (-s) on the client and the client mode (-c [IP of the client]) on the router. And vice versa.
iperf3 without any command will guide you.
Result
Hint
You can use the “reverse” switch -R on the client to test from router → client without need to change the server/client options. So .\iperf3.exe -c 192.168.177.1 -R will send the traffic from router to client.