@Shrimpy2836 I have a Speedport as well, but an older model (W724V Typ A). I tried the workaround as well, but in my router configuration menu (I accessed it through http://speedport.ip), they asked me to provide a password when I wanted to switch my DNS settings to the Cloudflare data. Did you have the same password request, and if so, did you set your personal router password there, or what did you do?
Changing the DNS settings only in the Windows ethernet settings didn’t solve the problem for me, so I guess that I need to change them in the router configuration menu as well.