After hours and hours of searching for a solution to my UDP port problem, I found a way around (because even tho I opened in router it didnt work).
This is what worked for me, it may not work for you.
First, you need to install skype. Then try to make a video call with someone to test if it works. See if both camera's show up, yours and theirs.
If yes, then it means that the UPD and TCP ports from skype is opened.
Then, what we gonna do is, were gonna use the UPD port from skype in Tennis Elbow. The port number is 10926.
So in game change the number 4321 to 10926, and host a server, let it test the connection and it should be fine.
Thats what I have done, and what worked for me, I wanted to share it with you guys.
(no raging at me if it doesn't work for you)