Apple has stopped signing iOS 12.1.2 and iOS 12.1.1. So if you want to downgrade from iOS 12.1.3 … well, you can’t.
This isn’t unusual. The tech giant commonly stops signing older versions of operating systems to encourage users to have the latest up-to-date builds for security and stability reasons. The last “stop signing” change comes two weeks after Apple issued iOS 12.1.3 with bug fixes and performance improvements.