"The port or device is already disconnecting."

You usually get this error when you have clicked 'disconnect' and the modem is disconnting and you clicked on connect to quickly, and the computer is still disconnecting when you tried to connect again.

Give the computer 30 seconds to a minute and try it again (or take a washroom/coffee break), and try again.

If the computer still hasn't disconnected, reboot and reconnect.

Make sure no other applications are using the modem at the same time.