You can achieve this by adding colliders to your object and camera.

 - preferably set the camera collider `isTrigger` on if the object is a rigid body.
 - You can use tags, layers and game object name to filter which object you want to operate on.

Here is the code:


 [RequireComponent(typeof(Collider))]
 public class FrustomColliderDetector : MonoBehaviour
 {
 //Gets called when it enters the camera collider
 private void OnCollisionEnter(Collision other) {
 if(other.gameObject.tag == "object-tag")
 other.gameObject.SetActive(true);
 }
 //Gets called when it leaves the camera collider
 private void OnCollisionExit(Collision other) {
 if(other.gameObject.tag == "object-tag") 
 other.gameObject.SetActive(false);
 }
 
 }