Maybe try to change quotation marks:
in before_title=”<h2 class=’rpbt_title’>”
And did you replace </h3> into </h2> in shortcode?
http://prntscr.com/p6oz7v
Try this:
[related_posts_by_tax before_title='<h3 class=”rpbt_title”>’ after_title=”</h3>” columns=”2″ format=”thumbnails” image_size=”post-thumbnail” title=”MORE NEWS” posts_per_page=”6″ taxonomies=”category”]
Use ‘ instead ” in before_title=”<h3 class=”rpbt_title”>”
Try
before_title
(string) Text or Html to place before the title. Default is <h3>.
after_title
(string) Text or Html to place after the title. The default is </h3>.
https://keesiemeijer.wordpress.com/related-posts-by-taxonomy/#before-title
