Cannot unlock Remote Desktop lock screen

Mac Pro 2013. No display via any port. Connected via Remote Desktop. Displays a black screen with padlock. Cannot unlock. Tried everything. I can SSH in. I've tried UNIX commands, keyboard shortcuts (it makes a noise on Control Shift Power) I downloaded a UNIX executeable called "sunshine-ard" but it will not run via SSH. I don't have a Thunderbolt cable to use in target disk mode. I can't think of a way to install new system as it's a propriety SSD. Now - the padlock. Is it definitely RD? What is the case is unable to be locked? It's always been like this. There's never been a firmware password. I'm stuck. Any suggestions much appreciated.