On routers and modems the DHCP range is usually not allocating the full range for IP addresses to hand out to devices. If I were you I would configure the pi-hole to have a static IP address on the pi-hole itself so it will be independent from your router's DHCP server. For example let's say
- the DHCP IP range is 192.168.1.20 to 192.168.1.254
- then you can pick for example 192.168.1.19 for your pi-hole
With that approach you will avoid IP address conflicts and the pi-hole should be fine. If you're using systemd on the pi-hole that is maybe easiest, here an example :
Another option is to use nmtui from NetworkManager