Head back to the quarantine zone. Soon you will discover that the corridor has been locked and more bandits are waiting on the other side of the gate. You have to use the same tactics as when you tried to get to Albert, i.e. use vent shafts under the floor. Retreat a little bit so the guards could not see you, find an entrance to the vent shafts and approach the thugs undetected. Select the Voice Synthesizer from the inventory, aim at any of the opponents and tell him to press the button on the wall.
The deceived bandit will not manage to press the button, because the real Harley Quinn comes. Wait until she finishes the conversation with Robin and tells her men to lower the gate. Do not come out from the vent shaft until Harley approaches Batman hidden inside. Only then, use a Team Takedown.
When you manage to stun Harley start attacking her thugs. In this combat use Team Attacks together with Robin. It is good to use them especially against tougher bandits. After the fight approach unconscious Harley as Batman, pick her up and get back to the quarantine zone. This is where you will see another cut-scene.
After seeing the entire cut-scene, as Batman enter the cell next to Robin. Wait until the scene "resets" and approach the ally again. Press the interact key/button to throw Robin into the cell prepared for Batman. Listen to a conversation with Alfred.
Leave the film studio, jump into the Batmobile and head toward the Merchant Bridge. Park the vehicle in the designated spot and wait until the last upgrade is installed (the one you did not pick last time). By the way, Alfred will inform you about a new side mission.