On this page of our guide to Layers of Fear 2 we have described in detail the bath and cable riddle, which has to be solved during the second act of the game. You will learn where the cable is and what to do with it.
You will reach the place, where you have to solve the puzzle after passing the long section, in which you have to avoid the flames. You should specifically find the brown door shown in the image.
Start by unfolding the curtain on the left side so that you reveal the bathtub with the dummy.
Now find the socket on the right wall and pull the power cord out of it.
Take the cable to the bathtub's vicinity. Drop it when it's hanging over the tub, so the dummy will be "electrocuted." You can already go through a well-lit "colorful" door.