Check your router's user manual for 'port forwarding'.
Most consumer routers - if not all - have the ability do what is called port forwarding. This means you put your camera (or whatever) behind your router like all your other home network devices but the router presents the camera to the Internet as if the camera was actually on the router's IP internet address. The key is for the camera to be set to a port that nothing else is using.
Quick 101:
An IP4 internet address looks like this: 64.122.321.54
That would be similar to the address of your router to the outside world in this example.
But on the inside, your router would have another address, like this: 192.168.1.1 (192.168.xxx.xxx is set aside for networks inside a firewall, as is 10.0.0.xxx).
And everything else on your network would typically have an address from say, 192.168.1.2 to 192.168.1.254
Those are IP addresses. However, there is something even more fine grained called a port. For instance a regular web page, http://wrongplanet.net is actually served up on port 80: which is the convention for a web page. So it's really wrongplanet.net:80 (or 104.25.180.27:80). Anyway, your camera also serves up it's web pages (images and video) on a port, on it's web address, on your inside network. Your router can be set to present that unique port as if it is that port of the router's facing the outside world.
So your camera's full IP + port address might be 192.168.1.5:8080 in this example. It would be presented by your router to the outside word as 64.122.321.54:8080 and accessible from anywhere on the Internet.
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“For small creatures such as we the vastness is bearable only through love.”
―Carl Sagan