v4r Welcome to the community!
We have made the changeover, which will be completed in 30 minutes. Restart your modem afterwards. If you have problems, reset your modem. Press the reset button on the back of the modem with a pointed object for at least 25 seconds.
Regarding your question about the static IP address. With the IP reservation you can assign an internal IP address to a MAC address. This device will always get the same internal IP, which is needed for port forwarding of your PiHole.
If you want an external fixed public IP address so that your PiHole is always reachable from the outside, I recommend a free DynDNS service that you can install on your PiHole.
Greetings
Daniele