Hello there, kim-brecklandaccountancy. 

Yes, the option to automatically set up reminder for invoice reminders is unavailable. You'll have to manually send a reminder once it's overdue. I sent a feedback to our engineers to have an option to set reminders for invoices. They might include it on the next updates. 

In QuickBooks Online, you might want to use recurring invoices. It helps when you have a lot of customers that you charge the same thing at the same time. You can make it on a Scheduled, Unscheduled, or Reminder basis. 

You can always post your questions here if you need more help.