Cannot get plugin to work yet..

Hi,

I am using this plugin with Custom Post Types UI and Site Origin Page Builder (and parent theme).

There are, I think, a few separate issues.
Firstly I can get this shortcode to display in SO textwidget:
[related_posts_by_tax]

This one is not rendered at all:
[related_posts_by_tax format=”thumbnails” image_size=”medium”]
which I think may be an issue with Page Builder.

So, I switched tack and tried adding a couple of things in functions.php:

// Return posts with post thumbnails for the thumbnail_excerpt format.
add_filter( 'related_posts_by_taxonomy_shortcode_atts', 'rpbt_thumbnail_exerpt_args' ); // shortcode
add_filter( 'related_posts_by_taxonomy_widget_args', 'rpbt_thumbnail_exerpt_args' ); // widget
 
function rpbt_thumbnail_exerpt_args( $args ) {
    if (  'thumbnail_excerpt' === $args['format'] ) {
        $args['post_thumbnail'] = true;
    }
 
    return $args;
}

or

add_filter( 'the_content', 'add_related_posts_after_post_content' );
function add_related_posts_after_post_content( $content ) {
 
    //check if it's a single post page.
    if ( is_single() ) {
 
        // check if we're inside the main loop
        if ( in_the_loop() && is_main_query() ) {
 
            // add your own attributes here (between the brackets [ ... ])
            $shortcode = '[related_posts_by_tax format="thumbnails" image_size="medium"]';
 
            // add the shortcode after the content
            $content = $content . $shortcode;
        }
    }
 
    return $content;
}

neither of which returned anything.
My custom post type has its own categories defined, which worked fine with the simpler shortcode.

Do you have any ideas?