The CPT has been registered fine, with the name 'qna' and slug 'advice'.
Viewing domain.com/advice correctly displays the archive-qna.php template.
Viewing domain.com/advice/custom-post-title also works correctly and displays using single-qna.php.
The problem comes when adding these custom posts to the standard post categories.
In that instance when viewing domain.com/advice/category results in a 404.
I think I may of found a work around, creating a qna post that matches each of the standard categories we're using, then using a conditional within single-qna.php.
However is there another way to solve this?