I tried redirecting 18321 as well but people still can't see my games. However I also tried redirecting all tcp and udp ports (1-65535) to my pc and when I do this it works, I can host. Although this is not exactly secure

I port scanned the machine while hosting a game to find out if it uses any other ports but the only extra one that comes up is 18321. So this doesn't really make sense to me but at least I have a way to host games.