rb.moveposition cant do diagonals

when holding 2 keys at once, the player will only move on one axis at a time. how can i make the player move diagonally?

code is here:

     private void ApplyMovement()
     {
 
         if (Input.GetKey(KeyCode.W))
         {
 
             rb.MovePosition(transform.position + (transform.up * movementSpeed * Time.fixedDeltaTime));
 
         }
 
         if (Input.GetKey(KeyCode.S))
         {
 
             rb.MovePosition(transform.position + (-transform.up * movementSpeed * Time.fixedDeltaTime));
 
         }
 
         if (Input.GetKey(KeyCode.D))
         {
 
             rb.MovePosition(transform.position + (transform.right * movementSpeed * Time.fixedDeltaTime));
 
         }
 
         if (Input.GetKey(KeyCode.A))
         {
 
             rb.MovePosition(transform.position + (-transform.right * movementSpeed * Time.fixedDeltaTime));
 
         }
 
     }

if its important the function is being run in fixed update and movement speed is 4. thank you!