To publicly host a server, you first need to open port 5029 in your router settings or ISP website. This will require you to have access to the password for either of these, so you may need to ask whoever owns the account to it. You can search this up online, usually something like "[isp name] open ports". For my ISP, Xfinity, you have to log into their website and change it there. Once you do this you can register in-game by hosting a server and changing the room from "Unlisted" so the game will tell the Master Server to register your server.