If you've been adding reminders with Siri using a phrase like "Remind me to…" without any mention of Due, you've probably been using a workaround by deleting Apple’s Reminders app from your phone.
In the past, this would cause Siri to route all reminder requests to Due. However, Apple has never explicitly supported such a way to use Siri with third-party apps. The fact that it worked at all was a happy coincidence. This behaviour seemed to have been removed by Apple in iOS 16.
Please review this document for the officially supported ways to use Siri with Due: Using Siri Shortcuts to Add Reminders to Due