Hello from the US!

I had the same problem. What I did was to create a shell script that I execute with setting bash as shell and changing the working directory. Since then it seems to work.

This is a non-production setup, so some security is lacking e.g. executing a shell script in the root directory.

Crontab entry:
10 9,10,11,12,13 * * * cd /root/ ; /root/speedtest.sh

Actually Shell script speedtest.sh:
#!/bin/bash
/usr/local/bin/speedtest-cli 2>&1 | /usr/sbin/ssmtp -vvv my-email@address &> /tmp/speedtest-status-msg.txt

I hope this helps. I am still testing but the first set of tests looked promising.