If information similar to the following is displayed, the iPerf server has been successfully started:

You must be able to ping the iPerf server IP address on your phone after it is connected to the SSID.
If the iPerf server IP address can be pinged on your phone connected to the SSID but the speed test still fails, run the iperf3.exe -s -p 5001 command to restart the server, and then perform the test again.