Nevermind, I found the solution myself. If you want do distribute your own IPv6 DNS servers using RDNSS, you will need to also configure custom IPv4 DNS servers.

Model: Archer VR200v Hardware Version: v1 Firmware Version: 0.8.0 0.19 v002f.0 Build 150820 Rel.31945n ISP: Deutsche Telekom Hello, I'm trying to figure out how I can influence which IPv6 DNS servers...