Laravel Eloquent：在null上调用成员函数connection（）

In Laravel I have a Controller like this:

<?php
namespace App\Http\Controllers;
use Illuminate\Http\Request;
use Illuminate\Database\Eloquent\Model;
use App\Dienstreise;

class DienstreiseController extends Controller
{
  public static function store()
    {
     $dienstreise = new Dienstreise;
     $dienstreise->user_id=1;
     $dienstreise->start_ort = "Hamburg";
     $dienstreise->ziel_ort = "Kassel";
     $dienstreise->save();
     return "OK";
  }
}

I have a route for that function, that works totally fine. (So the database-settings are 100% correct)

Route::get('/dienstreise', 'DienstreiseController@store');

But now I have a file in a folder in my public-folder, which needs to access that function in the controller too. (I know that sounds strange, but the file will get a POST Request from Dialogflow and so the file must be located in the public folder, otherwise they can't access it)

The file looks like this:

<?php
require __DIR__ . '/../../vendor/autoload.php';

use App\Http\Controllers\DienstreiseController;
use Illuminate\Http\Request;
use Illuminate\Database\Eloquent\Model;
use App\Dienstreise;

$dr = DienstreiseController::store();
echo $dr;
?>

But I get a 500-HTTP-Error? The log:

PHP Fatal error: Uncaught Error: Call to a member function connection() on null in /app/vendor/laravel/framework/src/Illuminate/Database/Eloquent/Model.php:1140

If I comment all the lines with $dienstreise... in the controller, I get the "OK", so the mistake must be there... But why does it work with the route? I'm confused. I read about uncommenting "$app->withEloquent();" in bootstrap/app.php, but there is nothing like this commented?