T-Bar from the Cable Car Platform
Use it to open the G-Adult’s section to obtain the Chess Plugs and Sewer Key. GET THE SEWER KEY.
After obtaining the T-Bar
Head back to the Lower Waterway to open up the Workroom Lift. This will allow you to use the sewer key to head back to the secret room.
With the Sewer Key
Open the Workers’ Break Room, move the yellow taped cabinet, go up the elevator, climb up all the stairs and go up another elevator. This should take you back to the secret room after completing the medallion puzzle. Use the T-Bar on the slot.
The last and most overlooked spot
Heading all the way back to where you first drop down to enter the main sewer areas from the Water Gate, it will be directly to the left in the Upper Waterway.