How to access repeater field of a custom field?

Try this:

foreach ( $donation_group_posts as $dg_post ) {
    $dg_id = $dg_post->ID;

    if(have_rows('YOUR_REPEATER_SLUG', $dg_id)){
        while(have_rows('YOUR_REPEATER_SLUG', $dg_id)) : the_row();
            echo get_sub_field('YOUR_FIELD_SLUG');
        endwhile;
    }

}