<?php

namespace app\models;

use Yii;

/**
 * This is the model class for table "products".
 *
 * @property int $id
 * @property string $name
 * @property int $supplier_id
 */
class Product extends \yii\db\ActiveRecord
{
    /**
     * {@inheritdoc}
     */
    public static function tableName()
    {
        return 'products';
    }

    /**
     * {@inheritdoc}
     */
    public function rules()
    {
        return [
            [['supplier_id'], 'integer'],
            [['name'], 'string', 'max' => 255],
        ];
    }

    /**
     * {@inheritdoc}
     */
    public function attributeLabels()
    {
        return [
            'id' => 'ID',
            'name' => 'Name',
            'supplier_id' => 'Supplier ID',
        ];
    }
}

Controller - ProductController

<?php

namespace app\Controllers;

use app\models\Product;
use yii\data\ActiveDataProvider;
use yii\rest\ActiveController;

class ProductController extends ActiveController
{
    public $modelClass = 'app\models\Product';
    public $serializer = [
        'class' => 'yii\rest\Serializer',
        'collectionEnvelope' => 'items',
    ];

    public function actionIndex()
    {
        var_dump('here');
    }
}

and routes in url manager

...
 'urlManager' => [
            'enablePrettyUrl' => true,
            'showScriptName' => false,
            'enableStrictParsing' => true,
            'rules' => [
                ['class' => 'yii\rest\UrlRule', 'controller' => 'product'],
            ],
        ],
...