<div class="modal-footer-demo">
<a href="" class="btn btn-primary" data-toggle="modal" data-target="#modal">Demo</a>
</div>$(document).ready(function(){
var inst = $.remodal.lookup[$('[data-remodal-id=modal]').data('remodal')];
inst.open();;
});
<style>
.modal.fade{
display:none;
position: fixed;
top:50%;
left:50%;
background-color: aquamarine;
padding: 30px;
}
.modal.fade.open{
display:block;
}
</style>
<div id="modal" class="modal fade">
<div class="modal-dialog" role="document">
<div class="modal-content tx-size-sm">
<div class="modal-body tx-center pd-y-20 pd-x-20">
<button type="button" class="close" data-dismiss="modal" aria-label="Close">
<span aria-hidden="true">×</span>
</button>
<i class="icon ion-ios-checkmark-outline tx-100 tx-success lh-1 mg-t-20 d-inline-block"></i>
<h4 class="tx-success tx-semibold mg-b-20">Поздравляем!</h4>
<p class="mg-b-20 mg-x-20"><?php echo $_SESSION['success']; unset($_SESSION['success']); ?></p>
<button type="button" class="btn btn-success pd-x-25" data-dismiss="modal" aria-label="Close">Продолжить</button>
</div>
</div>
</div>
</div>
<script type= text/javascript>
document.addEventListener("DOMContentLoaded", function(event) {
document.getElementById('modal').classList.add('open');
});
</script>