Previous/Next based on primary_category

Hey there
We try to use the plugin: Woocommerce Product Primary Category Plugin added with Previous and Next links. We tried the following code, but the sorting feels very random.

<?php previous_post_link_plus(array('link' =>'previous product', 'format' => '%link', 'order_by' => 'numeric', 'meta_key' => 'primary_category', 'in_same_cat' => true, 'post_type' => '"product"')); ?>
<?php next_post_link_plus(array('link' =>'next product', 'format' => '%link', 'order_by' => 'numeric', 'meta_key' => 'primary_category', 'in_same_cat' => true, 'post_type' => '"product"')); ?>

THANKS in advance for your help!
Marvin

Plugin used:
https://codecanyon.net/item/woocommerce-product-primary-category-plugin/19642775