Use Pi-Hole in different subnet

Installing and Using OpenWrt
2

In /etc/config/dhcp :

config dhcp 'guest'
[..]
        list dhcp_option '6,192.168.1.6'

in /etc/config/firewall:

config rule
        option target 'ACCEPT'
        option dest_port '53'
        option proto 'udp'
        option src 'GuestZone'
        option family 'ipv4'
        option dest 'lan'
        option name 'Allow-guest-pihole'
        list dest_ip '192.168.1.6'