Are you using the Asus router for DHCP ?
If so, check if Pi-hole is the only DNS server configured on the clients with below one for Windows clients:
ipconfig /all
And below one on Linux clients:
cat /etc/resolv.conf
I own an Asus RT-N66U and I cant use the Asus DHCP service as it will always push its own IP address as a DNS server to the clients regardless what I enter on the LAN-->DHCP server settings page.