Use the door marked on the above picture after a new day starts.
The door will lead to another room of the casino. Use the last door on the left to continue the task. If you're performing all the actions without resetting the day, you shouldn't encounter anyone there.
You will enter a corridor shown above. Use the marked door to enter the monitoring room.
Walk towards the list in the left corner of the room and interact with it. You will obtain codes that will enable you to turn the cameras on and off in the mansion.