You need to PORT FORWARD on your Router (you need to log on to the router to do this)
Each brand has its own way, so consult the users documentation
My last 2 brands, there were 2 sections to configure
A trigger (the port , protocol) eg 8080 TCP
A service (the host) (the IP of the Calibre PC)

Use the router to assign a
RESERVED IP, that way it will always get the same INTERNAL network IP (eg 192.168.0.12)
The router may allow additional restrictions: Hours, User IP (the person connecting PUBLIC IP, ... )