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Unless you are using ClearOS as a gateway, you should set the Network -> Settings -> IP Settings -> Network Mode to "Standalone" (https://example:81/app/network). This blocks everything.
Then visit https://example.org:81/app/incoming_firewall or via the admin panel Network -> Firewall -> Incoming where you can add some "Allowed Incoming Connections"
For example, if you are running a website, you should allow HTTP (port 80) and HTTPS (port 443):