The most common solution here is you need to manually Port-Forward your router to your host machine. You will need to google for specific instructions for your router to set this up. When manually port-forwarding for Run8, make sure to use UDP mode.

Run8 attempts to port-forward itself during host startup, however, depending on your router's settings (specifically UPnP), it may not be able to accomplish this.

Another less common cause of being unable to host is due to Carrier-Grade NAT (you can google that for yourself to learn about it). If your internet provider has you behind CG-NAT, there is unfortunately nothing you can do; someone else with a direct internet connection will need to host. This only affects hosting; if you're behind CG-NAT, you can still join someone else as a Client.