Dear @Jamyla
If the technician wasn't able to resolve the problem, then no money should have been charged..
The speed tests indeed show the appropriate results:
https://ziggobeta.speedtestcustom.com/result/4a26e070-34cb-11ec-941b-8796eb3e5efd
Therefore the actual download speed (both through cable connection and wifi) is not higher than 8Mbps, which is almost 40 times lower than the speed mentioned in the subscription. This speed is shown while downloading files through the browser, game clients, and connecting a gaming console to the modem directly.
The stability of the connection is not stable as well - the internet is freezing for several seconds occasionally.
As it was mentioned before by @tobiastheebe the data from ziggo box shows that there are some disturbances, which might cause those problems. On the other hand, the technician only run speed tests and made a factory reset of the modem, but did not say anything about the downstream and upstream values in the data I showed him. He only said that RxMER (dB) values are in range and are ok.
Unfortunately, I don't have knowledge of which values represent what, so I was not able to do anything but accept this answer.