Germany lets you reduce a broadband bill, or walk out of the contract, when the measured speed misses the contract badly enough. Three rules decide it, and only one of them has to be met. Put your own numbers in and see which ones you are already hitting.
These three numbers are on the Vertragsinformationsblatt your provider had to give you before you signed, and on your contract summary. They are not the same number, and only the first one is ever advertised. Download in Mbit/s.
The official Desktop-App wants 10 measurements a day on three different calendar days, with at least one clear day between them and no more than 14 days end to end. Enter the fastest and the slowest download it recorded on each day.
The second rule counts every reading, not just the best one of the day. Open the measurement report and count how many of the 30 downloads reached the normally available speed.
The report is produced by the Bundesnetzagentur's own Desktop-App at breitbandmessung.de. A browser speed test proves nothing here; the protocol from that app is the evidence your provider has to answer.
The three tests come from the Bundesnetzagentur's Allgemeinverfügung of 8 December 2021. The law sets the proportion of the reduction, not the arithmetic, so the euro figures above are the band an argument sits in, not a settled amount.