I think many modern routers requires you to do other things, I don't know much about it so I can't explain it, but I think it's called or related to DMZ, not sure though.
But I've had this problem many times over the years using different routers.
In my router I would have to assign the public IP to one of my machines in the router to make it work.
Try looking for "local devices" or something somewhere in the router interface, find the device that is your machine, and see if you can do something more than just port forward, like "assign public IP" or something else.
If you find your device and don't know what the options shown means, list every option here if you like.