A recent update to my router firmware has tuned up exactly that - Ookla speedtest directly from the router. A pleasant surprise addition.
Samknows box plugs direct into your router and uses various types of traffic to derive a fairly accurate speed profile of common usage patterns. Not essential in your case, as you have already isolated the problem to a local device, but it still produces some interesting statistics, particularly for different times of day. I wonder how the Olympics Games currently on will affect global traffic, most people having got used to consume media online rather than via TV during covid lockdowns and isolation?