Need help with portforwarding

[GamingBros]

My router will not portforward no matter what I do. But when I run the TerrariaServer.exe file and start a server, it opens up port 7777 no problem. I have no clue why this is, and would like an explanation of how I can open ports and make it work (I have the Linksys E1200 router wired to my desktop). I can upload pictures if needed.

 

[little_me]

well, it could be that you have port triggering enabled.
It automatically forwards ports in use to internal IP using them, allowing in theory, multiple computers to do their thing.
Manual seems cover port forwarding too: (on page 57)
http://downloads.linksys.com/downloads/userguide/E_Series_UG_E900Rev_3425-01486_Web.pdf

Basically, you need to know your computers internal IP and enter it to the page as directed, then it should work.