Just a small post for future reference that I struggled with getting port forwarding to work correctly on my Ziggo Ubee modem. I wanted to be able to access my curing chamber controller from the outside world but whatever I tried I couldn’t get the port forwarding to work.
The solution was super simple, instead of using your outside IP in the forwarding rule you have to put in
0.0.0.0. It will then match correctly and forward to the inside IP address.