Sort WP_Query alphabetically from MB Custom Table

I would like to make use of ordering for a records database using MB Custom Table. I have created a custom post type for the records and am able to display all data correctly in a php template.

Using the WordPress, ‘orderby’ arg does not work. My custom posts are just displaying in the order they were posted.

$args = array( 
    'post_type' => 'cemetery-records', 
    'posts_per_page'=>20, 
    'orderby'=>'last_name',
    'order'=>'ASC', 
    'paged' => $paged
);
$loop = new WP_Query( $args )

This makes sense to me because it is looking in the WordPress tables, not my custom table. However, I am having trouble finding documentation on how to order them differently based on the data in the custom table. Can you point me in the right direction?