I would like to use AdGuard Home (like PiHole), therefore I would like to configure a local address to be used as DNS server. but this is not possible as the deco app does not allow local addresses in the dns field, it shows up as invalid IP.

But I can configure a local DNS server on IPv4 via the DHCP settings. On the IPv4 Internet setting is even adviced to set it on the DHCP if I try to enter local DNS server, but there is no such option on DHCPv6.