Dns setup with 2 routers?

i have one router from my isp and another one (asus) in my room. i only want pihole to run on my asus router and not the whole network. Currently my asus router has the gateway to the main one and the LAN dns set to the pi hole and the WAN set to openDNS server. as this router is behind another one is this correct or should i put the pi-hole DNS address in to the WAN DNS as well?

ok so my 1st router from the isp is
my personal router is and my pi-hole is
so on my personal router’s wan do i have to change ths dns to as it is behind the 1st router?