Can't seem to get collision to work among my sprites

Hey guys - I'm working on a top down brawl game and i have this little ball in a circle shaped arena. I drew an arrow on top of the ball to see in which direction it's facing. The arena is a sprite i have imported (blank rectangle with a circle in the middle).

I added an empty game object as a child of this arena, gave it a rigidbody2d and a circle collider. My movable "player-ball" should only be able to move within that given circle (the arena). No the problem is, that the collision between player and circle edge does not trigger. Both objects have a rigidbody2d and the circle collider of the arena has "is trigger" checked...i'll post my code below - maybe someone can give ma some insight.

Thx for any help! Codes in C# btw ;)

 using UnityEngine;
 using System.Collections;
 
 public class Player_Movement : MonoBehaviour 
 {
 
     private Vector2 movementVector;
 
     private float movementSpeed = 8;
     // Use this for initialization
 
     private float heading;
     private Quaternion rotationToHold;
 
     private bool movingX;
     private bool movingY;
 
     public bool wallTrigger;
 
     void Start () 
     {
 
     }
 
 
 
     // Update is called once per frame
     void Update () 
     {
         
         movementVector.x = Input.GetAxis ("LeftJoystick_X") * movementSpeed;
         movementVector.y = Input.GetAxis ("LeftJoystick_Y") * movementSpeed;
         ///Moving the Player
         GetComponent<Rigidbody2D>().velocity = new Vector2 (movementVector.x, movementVector.y*(-1));
 
         ///turning the player
         float heading = Mathf.Atan2(movementVector.x*(-1), movementVector.y*(-1));
 
         transform.rotation = Quaternion.Euler (0f,0f,heading*Mathf.Rad2Deg);
 
 
         //movement checks in order to keep rotation after letting go of stick
         if (movementVector.x == 0)
         {
             movingX = false;
         }
         else{
             movingX = true;
         }
 
         if (movementVector.y == 0) {
             movingY = false;
         } else {
             movingY = true;
         }
 
 
         if (movingX || movingY == true) {
             rotationToHold = transform.rotation;
         } else {
             transform.rotation = rotationToHold;
         }
 
         ///check if Player is inside the arena and restric his movement if not
         if (wallTrigger == true) {
             movementSpeed = 0;
         } else {
             movementSpeed = 8;
         }
     }
 
     void OnTriggerEnter (Collider other)
     {
         wallTrigger = false;
     }
 
     void OnTriggerExit (Collider other)
     {
         wallTrigger = true;
     }
 }