Detect collision between player and ground

I'm still learning Unity and right now I'm trying to make my player able to jump. Of course I don't want my player to be able to jump on forever, so my idea was to only enable jumping when the player is in contact with a floor object. This is the code I have so far:

 public class PlayerController : NetworkBehaviour
 {
 
     public float speed;             // Player movement speed
     private bool grounded = true;   // Contact with floor
 
     private Rigidbody rb;
 
     void Start()
     {
         rb = GetComponent<Rigidbody>();
     }
 
     // Show a different color for local player to recognise its character
     public override void OnStartLocalPlayer()
     {
         GetComponent<MeshRenderer>().material.color = Color.red;
     }
 
     // Detect collision with floor
     void OnCollisionEnter(Collision hit)
     {
         if (hit.gameObject.tag == "Ground")
         {
             grounded = true;
         }
     }
 
     // Detect collision exit with floor
     void OnCollisionExit(Collision hit)
     {
         if (hit.gameObject.tag == "Ground")
         {
             grounded = false;
         }
     }
 
     void FixedUpdate()
     {
         // Make sure only local player can control the character
         if (!isLocalPlayer)
             return;
 
         float moveHorizontal = Input.GetAxis("Horizontal");
         float moveVertical = Input.GetAxis("Vertical");
 
         Vector3 movement = new Vector3(moveHorizontal, 0.0f, moveVertical);
 
         rb.AddForce(movement * speed);
 
         // Detect space key press and allow jump if collision with ground is true
         if (Input.GetKey("space") && grounded == true)
         {
             rb.AddForce(new Vector3(0, 1.0f, 0), ForceMode.Impulse);
         }
     }
 }

But it seems OnCollisionEnter and OnCollisionExit never trigger. So the player is still able to jump whenever he wants. Am I doing something wrong?

Edit: After some debugging it seems both OnCollisionEnter and OnCollisionExit work perfectly fine. It's the if statements returning false. I checked if the Ground object actually exists, just to be sure with:

 if (GameObject.Find("Ground") != null){ Debug.Log("ground found"); }

But that returned true just as expected. So there's something else going on.

Edit 2: Strangely enough both these seem to return "Untagged":

 Debug.Log(hit.gameObject.tag);
 Debug.Log(hit.collider.tag);