Setting Page Title Yoast

It sounds like you’re trying to modify document’s <title> tag as opposed to the post titles typically wrapped in <h1>/<h2> tags, which you’ve already accomplished.

If the theme you’re using implements titles in the WordPress 4.4+ manner, you can use the document_title_parts filter:

/**
 * document_title_parts
 *
 * Filters the parts of the document title.
 *
 * @since 4.4.0
 *
 * @param array $title {
 *     The document title parts.
 *
 *     @type string $title   Title of the viewed page.
 *     @type string $page    Optional. Page number if paginated.
 *     @type string $tagline Optional. Site description when on home page.
 *     @type string $site    Optional. Site title when not on home page.
 * }
 */
add_filter( 'document_title_parts', 'wpse241607_document_title_parts', 10, 1 );

function wpse241607_document_title_parts( $title ) {

    $title['title'] = '##### ' . $title['title'] . ' #####';

    return $title;
}

For Yoast SEO, you can check out the wpseo_title filter, which is used to filter the title text.