Internet connection behaving strangely across different networks on a Windows PC but flatmates's devices are fine

I have this problem for 1-2 weeks now, and only on my desktop computer.

The Problem:

I can play online games, I can listen to Spotify, the same wlan on my phone works perfectly. The only thing not working is all my browsers. Neither Chrome nor Mozilla or Edge work. And the speed is also terrible! I get maybe 2Mbit and should get 50-100Mbit when I load something via Steam.

But it isn't like the connection is completely unusable. I have the strange behavior that I can browse the internet like normal and normally fast (have a good internet connection, 125mbit/s) but suddenly everything stops working. No pages are loading and this persists sometimes for 10-15 minutes, sometimes for an hour. It's like 50/50 I can surf the internet or not.

What I tried:

Last month I was away from home on a different WLAN with my PC and it started in the last few days, and now I am back in my home network for almost a week and the problem continues.

Because the problem persists on several networks, I doubt the network is at fault here. Furthermore, I thought of any malware or virus, so I completely wiped my PC and installed a clean copy of Windows 10 so I doubt viruses or any software to be the culprit either.

I use a Netgear A6210 Wi-Fi adapter is not really old (half a year) and never was damaged or something like that, so I doubt that this makes the problems. I also updated my driver for it so that shouldn't be an issue.

Yesterday I called the support of my ISP and they could confirm that from the outside the connection is excellent, so they suspected it's a DNS problem. We tried to manually set the DNS, flush settings, and so on but nothing helped, and in the end he suspected that it might be a problem with the OS software aka Window 10. (Maybe something slipped through the new installation of the OS because I didn’t delete my data drives in the process?)

Furthermore I switched my network adapter (from USB to PCIe) and it is still bad. Plus I asked my other roommates if they had any problems and they did not have any so the router is also not at fault. I have absolutely no clue what this could be.

This is the speedtest on my phone (same wlan):

This is the speedtest on my pc:

This is the error i get when a site in chrome does not load: