Port is still close after portforwarding

I’m running a FTB Revelation server on my Synology-NAS and I can connect in the intranet, but when my friends or I want to connect with my public-ip, they can’t connect.

A portforwading tester says, that the port is closed.

I’m using a fritzbox and my ports are opened.

What could be the problem?

Source: Docker Questions