I finally found the weirdest solution to my problem. I have no idea why it works, but I changed nothing else but this setting.
Here is how I solved the issue:1. Log into your router interface
2. Open "Adaptive QoS".
3. Activate it (if not already activated).
4. Set the QoS type to "traditional QoS".
5. You have to set some value for the up- and download speeds (preferably one that is similar to the speed of your internet connection or do what I did and put a value that far exceeds my internet speed just to be sure).
6. Apply
7. After that the router loads and restarts.
I would be very interested to know why it works with QoS though.
If someone is able to explain to me why it works with QoS enabled, I would be very thankful
🙂