How do I add a custom function for a resource controller in laravel?

I'm trying to create a custom function for a Controller class that I'm using as ResourceController. I'm doing it cause I only want to update 1 field, but I want to keep the existing update function for other things.

I've tried to put it before the resource in the api.php as people said but still no good.

Route::put('/post/reorder/{id}', 'PostController@reorder');
Route::resource('/post', PostController::class);

This is the code in the controller

public function reorder(Request $request, $id){

        $post = Post::findOrfail($id);
...

Also, when running php artisan route:list it shows this error.

   Illuminate\Contracts\Container\BindingResolutionException

  Target class [PostController] does not exist.

This is the code in VUE that's calling it

export const API_URL = "http://localhost:8000/api/post";

axios
        .put(API_URL + `/reorder/${item.id}`, {
          // title: item.title,
          // path: item.path,
          // description: item.description,
          order: newIndex,
        })

And when I run it, it gives the Internal Server Error 500

It works when I'm using the default function that resource gave me, I just don't want to have to input the fields I don't want to change.

So what did I do wrong?