Laravel 5.4 how to get Articles related to a Category

I'm building a simple shop catalog in Laravel 5.4. Basically, what I want is to have a button on my article that gets all articles with the related categories (noting that an article can have many categories).

I have my Article and Category models, as well as an article_category pivot table. Both have a belong to many relationship to the other, like:

Code in Article model:

public function categories()
{
    return $this->belongsToMany('App\Category');
}

Code on Category model:

public function articles()
{
    return $this->belongsToMany('App\Article');
}

My issue is how to get the list of articles that include all articles with categories related to an article.

This is my function on the controller right now, it receives a JSON object with a list of categories:

public function showRelatedArticles($category) 
{
    // Decode JSON to PHP array
    $category = json_decode($category);
    $collection;
    if(is_array($category)){
        foreach ($category as $cat) {
            $articles = Article::whereHas('Category', function ($query) {
                $query->where('category_id', $cat->id);
            })->get();
            $collection = merge($articles);
        }
    } else {
        dd('Not array');
    }

    return view('front', ['articles' => $collection]);
}

This returns an error:
BadMethodCallException
Call to undefined method Illuminate\Database\Query\Builder::Category()

I may have the wrong type of relation between my models, but this is the correct method of declaring a Many to Many relationship, no?

via Chebli Mohamed