@jacopof 

Port 65535 is often reserved and not used for general network communication. It's considered a "null" port and typically not associated with any specific service. Using it for port forwarding might lead to unexpected behavior or conflicts.

To avoid conflicts , I recommend you to choose port numbers for port forwarding that fall within the private port range (usually 49152 to 65534) or any other unused ports that do not conflict with reserved ports.

  @Byron1234 

Unfortunately I have a device that has been configured to work only with port 65535, that's why I want to open it. Why instruction says I can set between 1-65535 but either single 1 or 65535 does not work?