there's some tricks to open a port, "punch a hole" in a NAT router. the most common is to use ICE and a STUN server.
https://github.com/arturoc/ofxNice
uses libnice which allows to do that but it's pretty temperamental and it won't work with some routers. then you can also set up a TURN server which will reroute all your traffic if the STUN server fails to open the router ports.
using upnp is also a possibility, it's a protocol to tell a NAT router to open a port from an application but most routers come with it disabled by default for security reasons. libnice/ofxNice internally can also do upnp.
in any case it's kind of complex to setup so unless it's for an application you plan to distribute to non technical people or something where you need to install the application in a lot of different networks or don't have access to the router config it's usually easier to just open the corresponding ports in the routers.
