Hello,
You can connect to the router via CLI and install additional package through OPKG for speedtests, exact commands should be ran in order:
1. opkg update
2. opkg install speedtest-netperf
after the installation you can run:
speedtest-netperf.sh -t 10
And it will speedtest the connection.
Please, keep in mind that this is third party package and it may have its own nuances.