Moving an Enemy randomly

I think the basic idea would be to calculate a random direction, move the enemy a certain amount of time in this direction and after that time, recalculate the random direction. You can have a more complex movement pattern, but this should suffice for the start.

So, if a certain time is over or if the enemy hit the wall, change the random movement direction. Apply the direction in in relation to the passed time since the last frame. I haven't coded 2D games with unity myself and I haven't tested the code, but this should give you a basic idea:

 private float latestDirectionChangeTime;
 private readonly float directionChangeTime = 3f;
 private float characterVelocity = 2f;
 private Vector2 movementDirection;
 private Vector2 movementPerSecond;
 
 
 void Start(){
     latestDirectionChangeTime = 0f;
     calcuateNewMovementVector();
 }
 
 void calcuateNewMovementVector(){
    //create a random direction vector with the magnitude of 1, later multiply it with the velocity of the enemy
     movementDirection = new Vector2(Random.Range(-1.0f, 1.0f), Random.Range(-1.0f, 1.0f)).normalized;
     movementPerSecond = movementDirection * characterVelocity;
 }
 
 void Update(){
     //if the changeTime was reached, calculate a new movement vector
     if (Time.time - latestDirectionChangeTime > directionChangeTime){
         latestDirectionChangeTime = Time.time;
         calcuateNewMovementVector();
     }
     
     //move enemy: 
     transform.position = new Vector2(transform.position.x + (movementPerSecond.x * Time.deltaTime), 
     transform.position.y + (movementPerSecond.y * Time.deltaTime));
 
 }

In OnCollisionEnter2D or whatever collision function you are using, you can then check, if the enemy hit a wall, if so, calculate a new movement direction. You could check for the wall by assigning a tag to the wall objects and compare the tags. If the gameObject of your collider has the tag "wall" you can then call the calcuateNewMovementVector() function again.