Script not working (Function not changing value)

I have this script that I made for an FPS character.

public class Player_Movement : MonoBehaviour

{ public CharacterController controller; public Transform camera;

public float gravity = -9.81f;

public Transform groundCheck;
public float groundDistance = 0.4f;
public LayerMask groundMask;
bool grounded;

public GameObject joystick;
public bool isMoving;
public float speed = 12f;
public float turnSmoothTime = 0.1f;
float turnSmoothVelocity;

public float jumpHeight;
public Vector3 yPosition;
public bool jumpAble = true;

// Update is called once per frame
void FixedUpdate()
{
    grounded = Physics.CheckSphere(groundCheck.position, groundDistance, groundMask);

    if (grounded)
    {
        yPosition.y = -2f;
        jumpAble = true;
    }

    Vector3 direction = new Vector3(joystick.GetComponent<Joystick>().inputDirection.x, 0f, joystick.GetComponent<Joystick>().inputDirection.y);
    isMoving = direction.magnitude != 0 ? true : false;
    
    if (isMoving)
    {
        controller.Move(direction * speed * Time.deltaTime);
    }

    yPosition.y += gravity * Time.deltaTime;
    UnityEngine.Debug.Log(yPosition);
    controller.Move(yPosition * Time.deltaTime);
}

public Vector3 Jump()
 {
     UnityEngine.Debug.Log(jumpAble);
     UnityEngine.Debug.Log(grounded);
     if (jumpAble && grounded)
     {
         jumpAble = false;
         yPosition.y = Mathf.Sqrt(jumpHeight * -2 * gravity);
         UnityEngine.Debug.Log(yPosition.y);
         UnityEngine.Debug.Log(yPosition);
         return yPosition;
     }

     return yPosition;
 }

}

The game operates on mobile touch controls, and the Jump() is called when the jump button is pressed. However, the character does not jump when I press the button, even though the function is being called, and the yPoisition is being changed in the function. I know that changing the yPosition should make the player jump, as when I add this code to the Update:

if (Input.GetKey(KeyCode.Space) && jumpAble)
         {
             fallDown.y = Mathf.Sqrt(jumpHeight * -2 * gravity);
         }

Everything works fine. Why is the new yPoisition value not being passed to Update, and why is this happening?