Tower 1
This tower is located on the rooftop of a building. To reach it, you have to go to the back alley and use a forklift from the building across it so you can climb it and make your way to the ctOS tower building. See the video below for more information.
Tower 2
Head inside the fence until you find a security cam. Control that cam and look to the left. Hack the forklift to lower the payload and to reveal another camera. Switch the control to that camera then hack the switch console to unlock the gate. Enter the gate, climb the ladder and head inside to unlock the tower using the console beside the forklift.