Zend certified PHP/Magento developer

How can I open a port on my public IP with my router?

I am trying to port forward a port on my IP. Unfortunately, my router (Sagemcom F@st 5657 interface) only lets me open ports on my private IP, which is useless if I want to connect from other computers. I have DMZ.

I checked if the port was open on https://ismyportopen.com/, and it wasn’t, which is obvious because I can only use my public IP to check.

How can I open a port on my public IP?