How to move an object in y-axis?

I have made a "Gravity Changer" that if I collide it with the player GameObject, it will change the gravity from (0, -9.81, 0) to (9.81,0 ,0). However the player cannot move horizontally in the new set gravity. When I press the default a and d button, it instead goes upwards or affecting the player's vertical speed. How can I make it able to move along the y axis with pressing a and d button?

Here are some scripts written (some may not be relevant):

     public float speed;
     public float jumpHeight;
     public float brakeFactor;
     public GameObject rocketBoost;
     public float boostSpeed;
     public float boostDuration;
     public GameObject gravitySwitcher;
     public GameObject camera;
     
     private Rigidbody rb;
     private bool isFalling = false;
 
     void Start ()
     {
         rb = GetComponent<Rigidbody> ();
     }
 
     void Update ()
     {
         if (Input.GetKeyDown (KeyCode.Space) && isFalling == false) 
         {
             GetComponent<Rigidbody> ().AddForce (Vector3.up * jumpHeight, ForceMode.VelocityChange);
         }
         isFalling = true;
     }
 
     void OnCollisionStay ()
     {
         isFalling = false;
     }
 
     void FixedUpdate ()
     {
         float moveHorizontal = Input.GetAxis ("Horizontal");
         float moveVertical = Input.GetAxis ("Vertical");
         
         Vector3 movement = new Vector3 (moveHorizontal, 0.0f, moveVertical);
         
         rb.AddForce (movement * speed);
 
         if (Input.GetKeyDown(KeyCode.S)) 
         {
             rb.AddForce(-brakeFactor * rb.velocity);
         }
     
     }
     void OnTriggerEnter (Collider other)
     {
         float moveHorizontal = Input.GetAxis ("Horizontal");
         float moveVertical = Input.GetAxis ("Vertical");
         
         Vector3 movement = new Vector3 (moveHorizontal, 0.0f, moveVertical);
         if (other.tag == "RocketBoost") 
         {
             Destroy (rocketBoost);
             rb.AddForce (movement * speed * boostSpeed);
             boostDuration = 3 + Time.time;
             if (boostDuration <= 0)
             {
                 rb.AddForce (movement * speed);
             }
         }
         if (other.tag == "GravityChanger") 
         {
             Destroy (gravitySwitcher);
             Physics.gravity = new Vector3 (9.81f, 0f, 0f);
             camera.transform.rotation = Quaternion.Euler (0f, 0f, 90f);
             float moveUpward = Input.GetAxis ("Upward");
             movement = new Vector3 (0.0f,moveUpward, moveVertical);
             rb.AddForce (movement * speed);
         }
     }