Before you start make sure you setup a static IP first:
How to setup a static IP address in Windows 10
portforward.com
Once you have done that succesfully, connect to your router's login page in your browser via the URL tab by typing in the login page's IP, which is usually 192.168.1.1 or 192.168.0.1.
Once you are there login with the credentials provided and port forward using your router instructions (In a manual or online).
The port should be 5029, as that is the default port used for SRB2.
Make sure when you port forward, you use the same static IP before for the port!
You should open a server in the game and use
https://www.jameds.org/SRB2/port/ (Thanks A'Dilla!) to see if you are alright! If you are then you are good to go, if not.
Follow these instructions for simply just port forwarding via Windows Explorer:
DISCLAIMER: This method is not guaranteed to work on every router. Consider it an alternative to this guide. Hey kiddies. Are you confused about port forwarding? Can you not get access to your router's wifi page? Did your router, for some...
mb.srb2.org