Do you know how to access your speedport router (I only have experience with FritzBox)?
This shows how to do it in German:
https://telekomhilft.telekom.de/t5/Festnetz-Internet/Speedport-Smart-Router-DNS-Server-aendern/td-p/2812191
It’s from 2017 so it might look different a little bit on your screen. They are configuring the Google DNS to IPv4 only. So maybe in 2017 IPv6 wasn’t available yet in the router’s software. I recommend changing both if you can.
If you want to only change it on your PC Ethernet Adapter, there’s some help farther up this thread or farther down on the link I posted.
Settings for IPv4:
Primary 1.1.1.1
Secondary 1.0.0.1
Settings for IPv6:
Primary 2606:4700:4700::1111
Secondary 2606:4700:4700::1001