If you want to change text You have successfully subscribed after getting a free membership and return it with a link, you can use this filter.
For example:
function custom_success_subscription_message($text)
{
$text = $text."<a href=".get_permalink(get_option('rtm-membership-page')).">".__('BACK','rtmedia') ."</a>";
return $text;
}
add_filter('rtm_membership_success_subscription_free_message','custom_success_subscription_message',10, 1);