You need to use NAT, so any external port you would assign to this, to be directed to the port 80 of the webif (and 8080 if you want also streaming the video/audio, and in that case I think you have no choice than opening the external port 8080 of your Public IP).

The Firewall also must allow this traffic, some routers if you set the NAT also have the firewall automatically enabled, but the professional ones need to have it also setup.

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