I wanted to share with you what worked for me when I could not host games.
Now you must have read somewhere that you have to open ports in rooter, but there is also another way for this.
Instead of opening a port in your rooter, just change the portnumber in your game to a portnumber that already is open.
So in game the port is set to 4321, what I did was I changed it to 27015. I know that the port 27015 is open because I played games like Counter-Strike and they use 27015.
So if you still have problems with hosting, try to change the port in game to 27015 like I did, and if it still doesn't work, find out which portnumbers are open in your rooter and use one of those in game.
I hope this helps.