Internet problems between PC and router?

I have the following problem I finished a PC for my sister and tested everything went well (I have a Fritzbox router).

Now she is at home and we have the problem that the internet is very slow and YouTube videos are not loading (tried several browsers). But on Steam in the download it is as fast as it should be. Everything is normal on the friend's laptop.

Now I've tried a lot, even a direct LAN cable from the 02 router to the PC but always the same problem.

I have already tried various WLAN sticks. As I said at home, it works with LAN and WLAN stick.

If you use the cell phone to connect the WLAN to the PC via USB, everything works great. The same if you connect to the LAN via the laptop and open a personal hotspot with the WLAN, you have no problems with the Internet on the PC.

As it seems to me, the PC can't communicate directly with the router. IP addresses etc. All obtained automatically.

Hope someone can help me.

PS: I've restarted the router and the PC too.

Times look in the router whether and how the priorities are set.

Also https://www.pcwelt.de/...g%E2%80%9C. Are a few tips.