Autocomplete on multiple fields Entity Reference
You can use the functionality Views: Filter by an entity reference view that comes with the Entity Reference module to search on more fields.

To use this functionality you need to change Entity selection mode to Views: Filter by an entity reference view instead of Simple (with optional filter by bundle) on the settings for your entity reference field.



First off you need to setup a View that lists your product nodes. Then add a display of the type Entity Reference. 



You can exclude the field cnk from display but add it as a search field to the settings for the Format "Entity Reference List". If you don't exclude the field cnk from display it will show content from the field in the autocomplete widget.



No go back to the entity reference field and choose the view you just created under the settings for Views: Filter by an entity reference view.