The checkbox on the left of the Game Object name (Player Rig) and the right of the cube is for showing and hiding the gameObject. I want to be able to enable and disable that checkbox (NOT the MeshRenderer!)
You cannot make any GUI elements in the Inspector invisible. If you want to disable the game object, call SetActive(false); in a script attached to the game object.
Is this script running on the same game object that is supposed to get disabled? If so, that might be the reason why you cannot activate the game object again. Update() gets called only on active components (and game objects). Assign your script to a game object that is always enabled and connect the other game object whose state is supposed to get toggled with it. Do you know the [SerializeField] attribute? If not, look it up in the API.