WordPress WooCommerce Add to Cart only adds one product

I’m quite new to WooCommerce and I can not figure out this one.
I’m creating a custom API (based on user requirement) and I’m letting user login in Laravel using a post request like this:

public function login (Request $req)
{
    global $woocommerce;
    $v = Validator::make($req->all(), [
        'username' => 'required|max:255',
        'password' => 'required|max:255',
    ]);
    if ($v->passes())
    {
        $user = wp_signon(['user_login'=>$req->input("username"),"user_password"=>$req->input("password")],true);
        if (is_wp_error($user) || !is_user_logged_in())
        {
            return response(['success' => false, 'message' => "Invalid Username or Password."], 401);
        }
        $key = md5($user->ID . $user->user_login. (time() + 7200) . $user->email);
        $cookie = wp_generate_auth_cookie($user->ID, (time() + 7200), "auth", $key);
        $store = [
            'user_login'      => $user->data->user_login,
            'user_id'         => $user->data->ID,
            'user_nicename'   => $user->data->user_nicename,
            'user_email'      => $user->data->user_email,
            'user_url'        => $user->data->user_url,
            'user_registered' => $user->data->user_registered,
            'display_name'    => $user->data->display_name,
        ];
        Session::push($cookie, $store);
        WC()->cart->set_session();
        return response(['auth_token' => $cookie, 'success' => true, 'cart'=>WC()->cart->get_cart_for_session(), 'timeout' => 7200], 200);
    }
    return response(['success' => false, 'message' => "Required field(s) missing."], 401);
}

Now, whenever I add a product to cart, it only replaces the previous value, and only one item is added to the cart. When I login to my WordPress site and visit the cart, it shows error “Undefined index”, while the cart is empty. Here’s the code for adding item to cart:

public function add($productId)
{
    //WC()->session->set_customer_session_cookie(true);
    $cart_id = WC()->cart->generate_cart_id( $productId );
    $prod_in_cart = WC()->cart->find_product_in_cart( $cart_id );
    // Add the product only if it's not in the cart already
    if( ! $prod_in_cart ) 
    {
        WC()->cart->add_to_cart($productId,1,0,null,null);
    }
    return response([
        'user'=>WC()->session->get_customer_id(),
        'cart'=>WC()->cart,
    ]);
}

I know I’m doing something wrong but don’t know what. Any help will be greatly appreciated.

Edit

If it helps I’m using WooCommerce version 2.3