You don’t need to disable DHCP for PiHole.
First get the IP address of your PiHole. Next go to your modem, key in the username password and go to Advance Settings then LAN IP settings on the side. At the bottom you will see a section for DNS values. Set both options to statically assigned and put the IP address of your PiHole there.
Keep in mind if your PiHole goes offline for whatever reason your internet will stop working. If that’s the case, change the DNS options back to the default values on the modem.