Scripts must be inserted using wp_enqueue_script. This function allows you to define dependencies, so you can be sure they load after jquery. Like this:
wp_enqueue_script('your-script-name', 'full-path-to-your-script', array('jquery'));
Even better, register your script first. Anyway, look in the plugin code where it enqueues user scripts and make sure it adds the jquery dependency. Or, perhaps better, build a child theme and add the following to functions.php:
add_action ('wp_enqueue_scripts','wpse229008_add_scripts');
function wpse229008_add_scripts() {
wp_register_script('your-script-name', 'full-path-to-your-script', array('jquery'));
wp_enqueue_script('your-script-name');
}