movement with changing gravity,Movement controll with changed gravity

Hey, i'm pretty new to unity and got stuck... I want to create a game where a sphere is controlled using WASD and in certain situations, the gravity of the game is changed, so that the player has to controll the ball "in a 90° angle" where the ball is attached to the wall...I hope you get what i mean :D

to change gravity i used:

public class ChangeGravity : MonoBehaviour {

 public bool normal;
 public bool left;
 public bool right;
 public bool up;
 private float gravity = Physics.gravity.y;
 

 // Use this for initialization
 void Start () {

     if (normal){
         Physics.gravity = new Vector3(0, gravity, 0);

     }
     if (left){
         Physics.gravity = new Vector3(gravity, 0, 0);
     }
     if (right){
         Physics.gravity = new Vector3(-gravity, 0, 0);
     }
     if(up)        {
         Physics.gravity = new Vector3(0, -gravity, 0);
     }
 }

such that i get enable a certain gravity whenever i need it...

for moving the player I use this script:

 public class Movement : MonoBehaviour {
     public Rigidbody rb;
     public float speed = 2000;
     public Camera cam;
     public float jumpforce;
 
     
     // Update is called once per frame
     void FixedUpdate () {
         //Forward
         if(Input.GetKey("w"))
         {
             rb.AddForce(cam.transform.forward*Time.deltaTime * speed);
         }
         //Backwards
         if (Input.GetKey("s"))
         {
             rb.AddForce(-cam.transform.forward * Time.deltaTime * speed);
         }
         //Left
         if (Input.GetKey("a"))
         {
             rb.AddForce(-cam.transform.right * Time.deltaTime * speed);
         }
         //Right
         if (Input.GetKey("d"))
         {
             rb.AddForce(cam.transform.right * Time.deltaTime * speed);
         }
         //Jump (at the moment also in the air)
         if (Input.GetKeyDown("space")){
             rb.AddForce(0, jumpforce * Time.deltaTime * 100, 0);
         }
     }
 }

with a 360° third person camera and moving into the direction the camera is looking when pressing "W".

My problem is now:

if the gravity is changed to "right", how do i move the ball? When pressing "a" the ball should go "up"...

i tried this:

 if (Input.GetKey("a"))
         {
             rb.AddForce(-cam.transform.up * Time.deltaTime * speed);
         }

but this does not seem to work...

any ideas on how to fix this?

Thanks in advance for any help!!!