Use Nagios to monitor a as modem configured Telekom Speedport Smart 3' DSL-link, upload- and download speed.
Set dedicated warn and crit limits for upload and download and also collect performance data.
View the complete instruction on how to implement this in your network in part 1 of my blog post: https://blog.timhartmann.de/2022/03/12/speedport-nagios-monitoring/
The way the data are extracted from the Speedport is based on a script by aaonk6