Please check the following points
1. Whether you are a Premium subscriber. You need to subscribe to the Premium Service in order to display reminders.
2. Check if the "Link with Apple Reminder" switch is turned on in the "Calendar Settings". If it is off, please switch it to on.
3. Whether the application is allowed to access the Reminders App. If you have once denied access permission, please allow access from the Settings app.
4. Check if the reminder's due date is set. Reminders that do not have a due date will not appear on the calendar.
5. If the reminder is set to repeat, only the most recent date will be displayed (this is an Apple Reminder's specification).
6. If a reminder is set to complete, the reminder will appear on the completion date in Planner's calendar (this is an Apple Reminder's specification).