When opening your reminders App for the first time on iOS/iPadOS13+, you were presented with an option to “upgrade” the Reminders App. You will also have been presented with a warning that compatibility with devices running earlier versions of iOS would be lost.
If you clicked-though and accepted the upgrade, you triggered a one-way update for your account on Apple’s servers. Once triggered, you cannot return - and Reminders for iOS12 devices are forever incompatible with newer devices. Sorry, but for iOS12 and earlier, the Reminders App cannot now sync’ with newer devices.
Upgrading the Reminders app in iOS 13 or later - Apple Support