restricting post nav to category

Working with a Twentyfourteen child theme and need my next-post/previous-post navigation to stay in the post’s category but none of the fixes I saw online as well as on wordpress codex worked for me in this theme. I understand I need to modify the code in the inc/template-tags.php, but whatever I tried was ignored. I’d appreciate any help.

Here is the code:

if ( ! function_exists( 'twentyfourteen_post_nav' ) ) :
/**
 * Display navigation to next/previous post when applicable.
 *
 * @since Twenty Fourteen 1.0
 */
function twentyfourteen_post_nav() {
	// Don't print empty markup if there's nowhere to navigate.
	$previous = ( is_attachment() ) ? get_post( get_post()->post_parent ) : get_adjacent_post( false, '', true );
	$next     = get_adjacent_post( false, '', false );

	if ( ! $next && ! $previous ) {
		return;
	}

	?>
	<nav class="navigation post-navigation" role="navigation">
		<h1 class="screen-reader-text"><?php _e( 'Post navigation', 'twentyfourteen' ); ?></h1>
		<div class="nav-links">
			<?php
			if ( is_attachment() ) :
				previous_post_link( '%link', __( '<span class="meta-nav">Published In</span>%title', 'twentyfourteen' ) );
			else :
				previous_post_link( '%link', __( '<span class="meta-nav">Previous Post</span>%title', 'twentyfourteen' ) );
				next_post_link( '%link', __( '<span class="meta-nav">Next Post</span>%title', 'twentyfourteen' ) );
			endif;
			?>
		</div><!-- .nav-links -->
	</nav><!-- .navigation -->
	<?php
}
endif;