I am using an OpenWRT router together with the router from my ISP (Sunrise/UPC Switzerland). I put them in the same broadcast domain by connecting the LAN port of my OpenWRT router to the ISP's router, and I turned off DHCP on my ISP's router so that my OpenWRT router acts as the only IPv4 router in my network, and it worked perfectly.
But for IPv6, the ISP didn't provide an option to turn off DHCPv6 on the router, nor is PD available. Hence my downstream devices have to solicit an IPv6 address, default route, and IPv6 DNS directly from my ISP's router. This is acceptable but I am seeking a way for my OpenWRT router to announce itself as an IPv6 DNS for my network so that I can use the hostname alias, ad blocker, etc.
I tried to specify a ULA prefix in OpenWRT and set both the RA and DHCPv6 service for the LAN port to server mode so downstream devices can get a local address with the ULA prefix. But unfortunately, the downstream devices can't add the announced IPv6 DNS properly.
Could you please provide some clues on how to deal with the problem in my case? Thank you in advance!