Because this is an issue for many players, I have decided to put not only a guide for port forwarding here, but also an alternative which I recently tested myself.
Note: You can also just go here in terms of the port forwarding part, since the wiki goes more into details.

Port Forwarding:

1. Go here and find your router brand on the list. If you cannot find your brand on the list you can ask your Internet Service Provider directly if they support port forwarding or not. If they do not, please refer to the alternative method mentioned below.

2. Follow the instructions for your specific router, although for the most part they run as follows.
example


  1. Setup a static IP address on your computer.
  2. Login to your router.
  3. Navigate to the port forwarding section.
  4. Create a port forward entry of 5029 for SRB2.



3. Test and enjoy!

Alternative Method:

1. Download Evolve (or an alternative LAN program); also go through the basic sign-up process and etc.

2. Create or Join a Party

3. Start SRB2 and host a game without choosing to advertise your server (aka an internet game, since Evolve is a form of LAN hosting)

4. Have your friends join you using your IP inside your Evolve party, you can simply right-click and copy the IP address, and then paste it right into the SRB2 console after the connect command; "connect 123.45.67.8" for example

5. Test and enjoy!