You can use any DNS on the main host router, DNS Relay ON or OFF. If you don't input any DNS in to the main host router, and DNS relay is ON, the router will automatically use detected DNS from ISP. If you input DNS into the router with DNS relay ON, the router will use what you input however the host will be 192.168.0.1 for DNS on all clients. If you turn OFF DNS relay with custom DNS, the clients will see what ever custom DNS you inputed on the router.
No need to put any DNS on the APs as they will automatically pass DNS from the router to the clients. No need for additional process of this.