Port Forwarding wont work

So I set everything up on my old Thinkpad running the newest Ubuntu Server OS and I can play on the server within my own network. Now I want my friends to join, thus i tried to forward the port 25565 via my routers routerlogin.net site (Netgear R6120). I set up a Port forward everything and started the server in the webui, but my friends cant join. When I check the port on Open Port Check Tool - Test Port Forwarding on Your Router it says that the port is closed. How can I fix that?

Additional information: Firmware V1.0.0.84, connectet to a modem

Often ISPs utilise something known as CGNAT which means your IP isn’t actually yours, it’s a virtual one and (if this is the case) you’d have to request it to be turned off. They may or may not do it or they may charge you for a static IP.