Laravel 5.5 update method not working for request add feature

In the following code the extra added field is not updating in the database. But the dd($request->all()) gives the extra key value pair is added in the request.

protected function update(Request $request, $id)
{
  $user = User::find($id);
  $request->request->add(['net' => 'abc']);
  $user->update($request->all());
}

via Chebli Mohamed