Yet another Asus router? How many routers exactly do you have and running at the same time on the same network? And the Pi-Hole device is not even a DIY? Which is from where?
Please, you should have only
one router running and
one dhcp server on the same network, either router's dhcp or pi-hole's dhcp. You can't have multiple devices (router or pi-hole) all giving out dhcp leases, static reserved or not. Unless they are are on different range on the same network, but that is still not the standard way.
However, since you will be using pi-hole as the dns on the network, the pi-hole should just set a static ip. Do not think about getting static leased ip from other routers for it, period, no one will setup like this.
If your router is 192.168.1.1, then set pi-hole on 192.168.1.2, dhcp range can start from 192.168.1.10 to whatever you like.
You should disable router's dhcp server and use Pi-Hole as dhcp server
https://discourse.pi-hole.net/t/how...t-in-dhcp-server-and-why-would-i-want-to/3026