Could be Ip6 related
I could not change the DNS settings since it does not allow to add local adresses therefore DHCP route
Hmmm ... https://community.ziggo.nl/t5/Internet/IPv6-en-Pihole/m-p/1014875 it seems there is no obvious route which refs How to set up IPv6 for Pihole correctly? - #6 by jpgpi250 someone implies to block all incoming and outgoing ipv6 traffic
but does not solve resolving "pihole.lan"
It probably will lead to having to put the router in bridge modus and pushing a seperate server next to it.