Internet is shown in the speed test that it works perfectly but it doesn't really work?

I have had the problem that my internet does not work on my PC for about 2-3 weeks, although it should work. I can only do the functions with the Internet that require little Internet and when I get into a game (Fortnite) then I also have a 20-25 ping. I can open Discord without any problems and also talk to my friends without an internet error occurring and I also have a 25 ping in Discord. I can't log into other games or applications because then an internet error occurs and the websites sometimes load for a long time. I have already reset my PC 2 times and the error is still there although everything has been reset. I already have a new LAN cable, the cat 8 so it can't attach it and a technician was already here and checked the internet. I have already done a full virus test in the Windows settings. I've tried everything and already have the latest Windows 10 update and I don't know what I can do anymore, please help me. My Windows version is:

Version 10.0.19042 Build 19042

Perhaps the network card is defective, but it is extremely strange that the speed test works.

Thanks, I'll find out more