From your question I'm guessing that you either have an iOS device running iOS 13 and a Mac running an OS version older then 10.15 (Catalina) or you are running Catalina but an iOS version older than iOS 13. Apple changed the format for reminders in both iOS 13 and Catalina that's not backwards compatible. When you started up Reminders for the first time you received a message and probably automatically clicked past it (I know I did this summer) not realizing what you'd just done.
So, Apple has changed the data format for Reminders and you must be running iOS 13 and Mac OS 10.15 to use it. No, I don't know of any way to back out of this if you gave the okay to use the new format. At this time I'm not aware of any intent on Apple's part to make the older Reminders compatible with the new data format. All I can do is commiserate and tell you what I did...permanently. I switched to 2Do. It is the reminder app Apple should be shipping with Macs IMHO.