Unattended Access changes PIN
Yes, you need to keep the Connect app open on the remote machine if you don't want the PIN to reset. And you should also uncheck the option "Close when incoming connection is terminates" in Settings -> General on this remote machine if you want to keep the same PIN after you close your connection.
As for the PRO license, it is required only on the computer FROM which you connect. If you have any issues with licenses, please contact our support team, and they will help you.