There is a lock pick in Police Gizmo's ? office.
Turn the lever in the middle position.
Getting through the cell grating without a key (or a lock pick) is impossible. Is there any other way into the cell?
Wouldn't it be nice to have the firefighter pole (Firefighter mode) present also in the cell mode?
Notice what happens when you change the mode between Firefighter mode and other modes.
The floor opens with the pole, and a hatch comes out at the right. See?
You should tie that hatch to something to prevent it from closing. It will control the pole.
You could use the handcuffs that hang, symbolocically, on some kind of scale. (Police mode)
Switch the office to firefighter mode and tie the pole hatch on the right to the pole with the handcuffs.
Switch to the police mode.
Go to the Front of Town Gate to climb up the pole into the cell.