The keypad sits over the display board.
The clear window sits over the display board LED.
The 4 pin connector connects the keypad to the display board.
The 4 pin should be a common and then one wire for Up, Down and Mode.
You can trace the wires to see which trace goes to the Up Button, Down Button and Mode Button.
Closing the common to the pin makes the function happen.
For example, closing common to up makes it go up.
Peel off the membrane pad and show the back.
Put waterproof tape over the opening.
Shut off power to the heater until you can replace the keypad and bezel.

Generic Chinese Keypad, Bezel and Display.
My guess is 1 is common and 2, 3 and 4 are Up, Down and Mode, but you can verify.
The traces will be something like this.
A trace will go from a pin to a button.
Each button will also go to common.
