Speed tests in Mist on SSR devices can fail for various reasons. One such reason is instability over the wan link.

• Speedtest fails to complete successfully

The following messages can be seen in mist agent journal log ("128T-mist-agent.service.log" in the tech-support-info  bundle)

Oct 08 14:43:57.051693 t128_testing_tools_controller[25925]: 0: building upload report
Oct 08 14:43:57.052735 t128_testing_tools_controller[25925]: 1: no loaded latency measurements
Oct 08 14:43:57.060579 python3.6[25925]: __main__ - failed to run speedtest: Unable to run mach with user facing message: Speedtest failed to complete

• Check for high latency/packet loss on the wan links. Mist UI - Wan Edge - Insights Page might also have interface specific loss/jitter/latency information.
• Run manual pings from the SSR wan interface to internet (for example 8.8.8.8) using Mist UI testing tools to verify latency/packet loss values.
• Review the ISP connectivity over the wan link and fix the network latency/ packet loss issues.
• Contact Support for further assistance