Leumhs what antivirus do you use? Some antivirus softwares like ESET have built-in firewalls that will block hosting on games by default.
Also make sure you port forwarded correctly in your router settings and that you chose the right device.
Also make sure to read all of the guide properly. The host needs to be in-game to be joined.
Last thing that comes to mind does any of you have softwares like Hamachi or Radmin installed or running?