ARCore - Lean touch- Use Lean Custom Rotation script to limit the rotation only in Y axis

I am trying to limit the rotation only to the y-Axis i.e I want the object to only rotate around it self in y-axis. But I am not able to do this.
Please help to solve this issue.
Thanks in advance.

This is what the rotation looks like:

Use this script

public class Rotate : MonoBehaviour
{
public float rotationSpeed = 0.7f;

[System.Obsolete]
void OnMouseDrag()
{
    float XaxisRotation = Input.GetAxis("Mouse X") * rotationSpeed;
   // float YaxisRotation = Input.GetAxis("Mouse Y") * rotationSpeed;
    // select the axis by which you want to rotate the GameObject
    transform.RotateAround(Vector3.down, XaxisRotation);
  //  transform.RotateAround(Vector3.right, YaxisRotation);
}

Do rem to put a box collider on the game object u want to rotate else it would not work.