Subtitle for Posts

Hi!
I would like to add subtitles to my posts but so far, no luck. I have found two plugins – Secondary Title, which works great but interferes with my breadcrumb navigation, and WP Subtitle. The latter requires using this code:

do_action( 'plugins/wp_subtitle/the_subtitle', array(
    'before'        => '<p class="subtitle">',
    'after'         => '</p>',
    'post_id'       => get_the_ID(),
    'default_value' => ''
) );

How would I put that below the inner page title in Materialis? Any advice?
Thanks a lot!
Kristyna