Sibling comment looks interesting, but two other options I've used are A) https://testmy.net/auto https://testmy.net/auto which lets you set up scheduled speed

Sibling comment looks interesting, but two other options I've used are A) https://testmy.net/auto https://testmy.net/auto which lets you set up scheduled speed tests, and B) some manual hacky scripts running via cron on a Raspberry Pi. The latter is useful because you can run whatever combination of tests you want - there are CLI programs that run on a Pi for bandwidth/speed testing (I think speedtest.net has one specifically, but there are others, including at least one other that targets speedtest.net), mtr/traceroute/ping/etc. etc. can all be scripted, and if you get a bit clever with your pipes and sed scripts, you can end up with a file that can be directly loaded into a spreadsheet program for plotting (or you could go all-out and have the script generate a plot.ly plot directly). I've also plotted ambient temperature here, to maybe find any correlations. What might be the most useful for you is to find a metric on your router/modem that provides some value - years ago I was able to find a signal quality metric on my DSL modem that I could record programmatically (simple API call with a the simplest HTTP password encryption option...), which eventually lead to discovering that there were line filters/attenuators that had been installed but not removed for the different seasons.