In this video, I install iperf3 network speed testing software on an iPad. The procedure is the same for iPhone.
iPerf3 software: https://itunes.apple.com/fr/app/he.net-network-tools/id858241710
iPerf Playlist: https://www.youtube.com/playlist?list=PLErU2HjQZ_ZNBUQCqkstfe7_zAdt7b1_k