I've tried setting a port open for a game a few times for my computer and port checker says its closed/ unreachable everytime. I've even tried DMZ mode which showed open for about 5 seconds then showed closed again. I even tried setting a different pc to DMZ to see if something on my desktop was blocking it. Same issue showed that port was closed. Port is 25565 which is used for minecraft so I can't imagine that poses a security risk and would be blocked by something on the backend.