If you actually did port forward correctly, it might be your firewall stopping SRB2 from hosting, also there is this thread
http://mb.srb2.org/showthread.php?t=42720 which has port forwarding information. This is a common question, i feel how it is not be able to host a server, but i found out myself, my computer had this unique problem, i port forwarded with the ipv4 adress that shows in cmd, and i hosted fine, but when i replaced the PC's board( an internal component because of a problem i was having with USB drives) it wasn't working anymore, CMD showed the same IP adress, but when i downloaded the port forwarding tool, it showed what was my real IP which was totally different, and i tried that IP and it worked. This was probably caused by switching of the computer's board.
What i am trying to say is that it might be your firewall, or you port forwarded with the incorrect IP.