To reach the first ctOS box, you have to see it using the camera installed on a house to the northwest. Once the position of the four boxes are revealed, go to the nearest one and look for the camera installed in the corner of the building across it. Hack that camera so you can see and switch controls to the next camera beside the box.
Next, head to the eastern box. The camera is installed in the corner of the building nearby with the construction lift beside it. Head to the last two boxes next. The box on the rooftop can be seen by another corner camera, the one installed in the apartment building beside where the box is located. The last box can be hacked using the street camera across it.