Based on my understanding of the query,
If you have a static public IP or DDNS name, you can access it by directly entering the public ip on your browser, but the router should have "HTTP Remote access enabled" in the System>Administration>Access control.
If you want to access a particular interface connected to your router like a camera or a hotspot interface you should have "Port forwarding" done from the Public IP, to the IP address and required port of the interface (Network>Firewall>Port forwards>Add New Instance)
Here,
External Port : The port you will be using to access the interface (Lets take 54 as an example)
Internal IP: The IP address assigned by the router to the interface (It will be in the drop down menu or you can add a custom one)
Internal Port: The port the interface is using (For example lets take 81)
Now when you access the router using the public IP and the assigned port ie. 84.xxx.xxx.xxx:54 in this case, it should redirect you to the required interface.
Hope this helps !