I own the XBOX one s and was actually used to having bad internet and a low download / upload speed (since I only had a 16mb line).
Now we have upgraded to a 100mb line that works perfectly (on all devices always at least 80mb) except the Xbox, where the download speed is only 20mb and upload 3mb.
I've read many times that the DNS address should be changed to 8888 and 8844, which didn't help either. Is there any other way to improve download speed?
If you now had a LAN cable as an idea: there's no router / amplifier in my room, which makes this impossible
Try putting the XBox closer to the router to see if that improves speed. Of course if the possibility exists.