Hey,
Looks like in your modem should reserve an IP address for your rpi. Generally I would always reserve over statically assigning it on the device. Here is how I did that on the xfinity gateway.
So now, assuming your starting fresh with a fresh install Rasbian, go ahead and install pihole. After that you just assign the dns to your devices. There are a couple ways to do that found here
You will start to run into problems acquiring the DNS server if you do not flush you devices DHCP cache properly. In windows I disable and turn back on the adapter in the adapter options for the network. But for any device just rebooting will give you the new DNS. Keep in mind there's nothing wrong with using your modems DHCP server. Unless there are features on the pihole DHCP you need, stick with your modems defaults!
After you get that running you can still use cloudflare's DNS before cascading to the pihole DNS.
