Hi and welcome to Ship sim forum.
First you need to know what your Router IP is, Then run something called "cmd.exe"
When it's opened, Type in IP Config. Your Router IP should appear somewhere.
Open your default browser, and type your Router IP Adress into console.
Now comes the weird part. I dont know what kind of router you use, and therfor. the pages are very different.
You should come to a part called Game Application or Port Forwarding
However, I will show you the way to do it if you've got Speedtouch router.
1: Press At Toolbox button, Then press the Game & Application Settings
2: Create a new game or application
3: Give it a name, example: SSEMP1, and then press Manual entries of port maps.
4: It should look like this:
Now Press Add.
5: Now repeat step 2 - 4, But at step three,change it to "2"
and at step 4 you should change the Protocol to UDP
6: TADAA! Your ports should now be working