I have some suggestion to improve the prerequisite's guide
How I read multiple times, PiHole born to be used on and inside local network so I think the security could be improved.
I know that if a port is not open on the router firewall it should be closed from the outside but there are a lot of low budget, not reliable router in the world, so my tips.
Not concerning security: the installation guide say that pihole-ftl use port 4711 on the loopback interface, but the pihole installation open ports between 4711 and 4720, so something is wrong;
Due pihole is used in local network I suggest do modify the guide suggested firewall rules as following
-A INPUT -s 127.0.0.0/8 -p udp -m udp --dport 53 -j ACCEPT -A INPUT -s 127.0.0.0/8 -p tcp -m tcp --dport 53 -j ACCEPT -A INPUT -s 192.168.0.0/16 -p udp -m udp --dport 53 -j ACCEPT -A INPUT -s 192.168.0.0/16 -p tcp -m tcp --dport 53 -j ACCEPT -A INPUT -s 192.168.0.0/16 -p tcp -m tcp --dport 80 -j ACCEPT -A INPUT -i lo -p tcp -m tcp --dport 4711:4720 -j ACCEPT
Have a good day, you are doing a wonderful work with PiHole!