More details later, but in a nutshell
Remove SSH (usually port 22) and Webconfig (port 81) access from default Incoming Firewall (https://example.org:81/app/incoming_firewall) and replace by rules in the Custom Firewall
Be careful not to lock yourself out!
iptables -I INPUT -p tcp --dport 22 -j DROP
iptables -I INPUT -p tcp --source 203.0.113.0 --dport 22 -j ACCEPT
iptables -I INPUT -p tcp --dport 81 -j DROP
iptables -I INPUT -p tcp --source 203.0.113.0 --dport 81 -j ACCEPT
Make sure you have activated the rules on the Custom Firewall (you disable a rule instead of deleting)