To have the ability to host successfully you need to have one and ONLY ONE of the following.
Correct port forwarding Rule
Port 28000 UDP+TCP forwarded to YOUR internal IP address (found by command prompt -> ipconfig).
If you already have this, check the internal IP address, if you are using DHCP this may have changed.
UPnP Enabled
By far the more simple option, also gives you an accurate ping on server browser. Usually located under "advanced options" in your router.
Do not have both enabled; this causes conflict.