Why can't I access my app externally?

I installed node, deployed my app (vue.js / nuxt app) and used npm install. When I use npm start it uses nuxt start and I get the message OPEN http://localhost:3000, which seems to be correct because if I open a second terminal and use netstat -plnt I get

 will not be shown, you would have to be root to see it all.)
Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address           Foreign Address         State       PID/Program name
tcp        0      0 0.0.0.0:22              0.0.0.0:*               LISTEN      -
tcp        0      0 127.0.0.1:3000          0.0.0.0:*               LISTEN      13929/node
tcp6       0      0 :::22                   :::*                    LISTEN      -```

I also if I use ```curl http://localhost:3000``` in a second terminal the app seems to work. However, if I enter ```http://MY_IP:3000/``` or ```http://MY_IP/``` in my browers I get ```Unable to connect - Firefox can’t establish a connection to the server at MY_IP:3000.```

Sorry, I am still quite new to this -  what am I doing wrong here? Why can't I access my app externally? Thanks for any help!