Long-term load (15min avg) larger than number of processors: 1.3 > 1 This may slow down DNS resolution and can cause bottlenecks

Long story short, you cant advertise the Pi-hole IP as the only DNS server for your clients via DHCP with the stock Asus firmware.
Safest solution is to enable Pi-hole's own DHCP service and disable the DHCP service on the Asus router.
Below thread is full of info and alternatives: