Thanks Albert for the reply!!!
The cable is the built-in cable of the DNS320, I have try to swap the LAN port on router: same situation.
This screenshot show the home page of the DNS320 open from inside the lan:

The login information (user & pwd) are stored, but when i press the login button, I lose the connection.
It's very strange...
I have also updated the firmware on my router, but nothing.
Thanks