Does not return value with mutators in laravel 5.4

I have the following problem when using mutators to be able to generate a value that is inserted into the database:

My model has a field called order responsible for carrying the position of a menu, when I generate a new menu I want you to see max('order') and increase +1 .

Mutator

public function setOrderAttribute($value)
{
     $query = $this->where('parent', '0')->max('order');
     $this->attributes['order'] = $query + 1;
}

Note: I tried doing another mutator with Name and if it worked, only when I manage whole does not add the value I need, I also tried with

$this->attributes['order'] = 20;

and nothing.

What am I doing wrong?

Greetings

UPDATE: The model

class Menu extends Model
{
    use SoftDeletes;
    use Sluggable;

    protected $table = 'int_menu';
    protected $primaryKey = 'id_menu';
    protected $dates = ['deleted_at'];  
    protected $fillable = ['nombre', 'icono', 'parent', 'order', 'id_page'];
    protected $casts = ['order' => 'integer'];


    public function page()
    {
       return $this->belongsTo('App\Model\Intranet\Page' ,'id_page');
    }

    public function getChildren($data, $line)
    {
        $children = [];
        foreach ($data as $line1) {
            if($line['id_menu'] == $line1['parent']) {
                $children = array_merge($children,
                                            [array_merge($line1,
                                                ['submenu' => $this->getChildren($data, $line1)
                                                ])
                                            ]);             
            }
        }
        return $children;
    }

    public function optionsMenu()
    {
        return $this->where('enabled', 1)
            ->orderby('parent')
            ->orderby('order')
            ->orderby('nombre')
            ->get()
            ->toArray();
    }

    public static function menus()
    {
        $menus = new Menu();
        $data = $menus->optionsMenu();
        $menuAll = [];
        foreach ($data as $line) {
            $item = [ array_merge($line, ['submenu' => $menus->getChildren($data, $line) ]) ];
            $menuAll = array_merge($menuAll, $item);
        }
        return $menus->menuAll = $menuAll;
    }

    public function setOrderAttribute($value)
    {
        /*$query = $this->where('parent', '1')
                        ->max('order');*/
        //$this->attributes['nombre'] = Hash::make($value);
        $this->attributes['order'] = 1;     
    }

    public function sluggable()
    {
        return [
            'slug' => [
                'source' => 'nombre'
            ]           
        ];
    }
}