Customize your posts navigation to get posts in the same tag.
the_post_navigation( array(
'prev_text' => __( 'prev chapter: %title' ),
'next_text' => __( 'next chapter: %title' ),
'in_same_term' => true,
'taxonomy' => __( 'post_tag' ),
'screen_reader_text' => __( 'Continue Reading' ),
) );