The list of invoices and their balance are under Sales then Invoices.
An invoice is only reminded if it HAS BEEN SENT to the customer. This is deliberate: reminding an invoice the customer never received makes a formal notice the first they hear of it.
So start by sending what has not been sent, then turn on automatic sending on the Invoice transmission screen if you wish.
Automatic payment reminders are set on the All my settings screen: on or off, maximum number of reminders and minimum amount, for the company or for one entity.
A reminder goes out on the customer's channel and states the remaining BALANCE, with the invoice's payment link.
PlansIncluded in every plan.