I think by default the broker will listen to all interfaces, hence as long as you have network access with internet access you can access the broker. But yes you will probably need to go into your router to create a port forward rule in the firewall cine the outside world only sees the IP address to your router. Your route then needs to forward thos eports request to your raspberry pi IP.
You'll also need to make sure the IP of your pi is known to the router, this depends on your setup but you should be able to assign a static IP as well. This way your raspberry pi aways as the same IP address.
I'm also in the process of opening my mosquitto broker to the outside. It had been limited to localhost for all the services. But now I want to get real time feed back, and control from the web app I created. Above were the steps I had to do to allow the web page served by the raspberry pi to be accessible. I'm exposing a websocket listener on the broker (although I'm having issue with this now which I posted a question about) but I am able to connect to it from my web page from anywhere over internet.
