Set movement speed on collision,movement speed on collision

This may seem a silly question, but I have been searching and still got no answer. I have a scene like this; https://gyazo.com/c6c0515607fd36327607ba8888a0eb91

When the robot's foot game object collides with the treadmil, I want to make it to start moving. I am sure the collision is right, and I got no error on referencing the robot's foot game object (the script is on the treadmil).

 public class MOVEMENT : MonoBehaviour {

 private Rigidbody2D MYRIGIDBODY2D;

 [SerializeField]
 private float MOVEMENTSPEED;

 //Get robot
 GameObject ROBOT;
 private Rigidbody2D ROBOT_RIGIDBODY2D;

 // Use this for initialization
 void Start () {
     ROBOT = GameObject.FindGameObjectWithTag("robot");
     ROBOT_RIGIDBODY2D = ROBOT.GetComponent<Rigidbody2D>();
 }
 
 // Update is called once per frame
 void FixedUpdate () {
     
 }

 void OnCollisionEnter2D(Collision2D coll)
 {
     if(coll.gameObject.tag == "robot")
     {
         ROBOT_RIGIDBODY2D.velocity = new Vector2(MOVEMENTSPEED, 0);
         //Debug.Log(ROBOT_RIGIDBODY2D);
         Debug.Log("hit");
     }
 }

Is there something that I missed? If i set the velocity on the FixedUpdate, the robot's foot will move though. Can someone help me? ,