Laravel 5.1 Update data with relation

I want to update the model Chapters (table: Chapters).

Right now it does not update the table I get following error:

BadMethodCallException in Macroable.php line 81: Method product does not exist.

Here is a little scretch of the 2 database tables:

Chapters:

id         |       product_id       |     name
_________________________________________________
3          |             2          |     jon doe


Products:

id        |       ...
_____________________________________
2         |       ...

I want to update / add a chapter to my chapters table.

My update method:

    public function updateProduct(Request $request)
    {
        ...
        $productID = $data['id'];
        $product   = Product::find($productID);
        ...
        
        $chapters = $request->input('chapters', []);

        if(count($chapters) > 0)
        {
            $chaptersData = array();
            $o = 1;

            $inputChapters = Chapters::where('product_id', $product->id)->get();
            foreach ($chapters as $chapterKey => $chapterValue)
            {
                $inputChapters->chapter_name = $chapterValue['chapter_name'];
                # .. some more
               
                $chaptersData[] = $inputChapters;

                $inputChapters->product()->associate($product);
                $inputChapters->save();

                $o++;
            }
        }
        ...
    }

Edit my chapter model

<?php

    class Chapters extends Model
    {

        protected $table = 'chapters';


        protected $fillable = ['product_id', 'chapter_name'];


        public function product()
        {
            return $this->belongsTo('App\Product', 'product_id');
        }


    }