2D fixed cam aim problem

Hi my game camera is fixed , i have a script for gun aiming looks like this.

Vector3 mouse = Input.mousePosition; mouse.z = 0f;

         Vector3 screenPoint = TheCam.WorldToScreenPoint(transform.localPosition);
         Vector2 offset = new Vector2(mouse.x - screenPoint.x, mouse.y-screenPoint.y);

         float angle = Mathf.Atan2(offset.y, offset.x) * Mathf.Rad2Deg;
         transform.rotation = Quaternion.Euler(0, 0, angle);

this code is only usefull non fixed cams because mouse rotate is set to screen middle if my player go out of middle my gun aim is broken

i how to set my mouse rotation to my Player ? ( The camera is fixed and the player moves independently of the camera.)