I am having this same exact issue, I have 2 zomboid servers, one I dont use and Im letting go at the end of the month...the other is the one I need and it isnt showing up in host. The only difference is that I am not trying to get a backup file to show...I am just trying to get the server to show up. I have wiped the server, manually deleted any and all saves, reinstalled the game. The server is fired up and working...just not showing up in the host section so I can make a few modifications.