WordPress pagination with Bootstrap 4 and Grid content Display

That’s because you have to change your query also have add

'paged' => get_query_var( 'paged' )

So your new query should look like

 $args=array(
 'post_type' => 'post',
 'paged' => get_query_var( 'paged' ),// add this line 
 'post_status' => 'publish',
 'posts_per_page' => 20
);

For for information have a look at the WP_QUERY Class