Spawn enemies out of sight on top of navmesh

Question

I'm trying to spawn enemies out of camera sight, as long as they are on top of a navmesh. This is the code, but for the moment it does nothing. What am I getting wrong?

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.AI;

public class EnemySpawner : MonoBehaviour 
{
    private int waveNumber = 0;
    public int enemiesAmount = 0;
    public GameObject Enemy;
    public Camera cam;

    // Use this for initialization
    void Start () 
    {
        cam = Camera.main;
        enemiesAmount = 0;
    }

    // Update is called once per frame
    void Update () 
    {
        float height = cam.orthographicSize + 1;
        float width = cam.orthographicSize * cam.aspect + 1;

        if (enemiesAmount==0) 
        {
            waveNumber++;

            for (int i = 0; i < waveNumber; i++) 
            {
                NavMeshHit hit;

                if(NavMesh.Raycast(cam.transform.position, new Vector3(cam.transform.position.x + Random.Range(-width, width), 3, cam.transform.position.z + height + Random.Range(10,30)), out hit, NavMesh.AllAreas))
                {
                    Debug.Log("Hit");
                    Instantiate(Enemy, new Vector3(cam.transform.position.x + Random.Range(-width, width), 3, cam.transform.position.z + height + Random.Range(10, 30)), Quaternion.identity);
                    enemiesAmount++;
                }
            }
        }
    }
}