I previously used the AVM Fritzbox 6850 5G but they dont support Wifi 6 so i switched to Deco X50-5G. In the Fritzbox you can change the DNS and its called there DNSv4 / DNSv6. I found this page...

Please implement to change the WAN/IPv4 DNS if you dont trust your ISP or want faster internet you can change DNS to Google (8.8.8.8) or Quad9 (9.9.9.9) if you want to use AdBlock DNS via PiHole,...

@David-TP hey, will it ever be possible to change the DNS Servers for WAN/IP on the Deco x50-5g? right now its only possible to set DHCP DNS