Which settings on my router can I try out to improve the Wi-Fi signal or which problems could be?

I've got a 500 mbit line and maybe over 50 to 150 in the download (always very fluctuating) with ethernet cable but I apparently have no problems according to the speed test (connected via Xbox, downloads still take a lot longer than they should). I've also changed the router a few times, etc. But nothing helped!

Your device has a problem. Interference source nearby, USB 3 stick e.g.