That way the Asus router can cache DNS queries for all your clients without the DNS query leaving your fast LAN.
Pi-hole does same caching for your clients.
Other Pi-hole features like "Groups" wont work either if Pi-hole only sees the queries from the router.
No wont work ... not as long as clients query the router first.
None of the client queries will get through to Pi-hole, only the router ones.
As long as you configured the Pi-hole address as the only DNS server in the WAN/Internet DNS fields!
If you configured a second IP like for example your ISP DNS servers, ad blocking might not work when the router decides to use your ISP DNS thus bypassing Pi-hole.
Its not that complicated:
Make sure to enable the Pi-hole DHCP service first before you disable the one on the Asus router.
Tail the logs for DHCP:
tail -F /var/log/pihole.log | grep dnsmasq-dhcp
Grab a test PC/device and disconnect and reconnect it from network (or reboot it) to renew its DHCP lease while tailing the logs .
And test if the http://pi.hole site loads on that divice.
If dont work, just enable the DHCP service on the router again and disable the one on Pi-hole.
Dont forget to copy over DHCP reservations that you might have created on the Asus router!