The Dunning feature should work for normal transactions and also Shopify subscriptions. Of course these e-mails will only be sent to the customers when the invoice is overdue. If you want a simple payment reminder you have to work with the Charge Flow instead.
If you need further support could you please reach out to us via firstname.lastname@example.org and explain your exact desired process on how you would like the dunning flow to work ?
We will the analyse and try our best to help you set this up.