Apple has ceased signing iOS 12.1.1 and iOS 12.1.2 following the recent release of iOS 12.1.3.
You no longer have the opportunity to downgrade your iPhone, iPad, or iPod touch if you have already upgraded to the most recent update.
If iOS 12.1.3 has been buggy for you, tough luck. The small window you had to perform a downgrade has now been closed, which means you’re now stuck with this release until a new one arrives.
Downgrading to iOS 12.1.1 or iOS 12.2.2 no longer possible
This means iOS 12.1.3 is currently the only public version of iOS that can be installed on an iPhone, iPad, or iPod touch. Once Apple stops signing a release, there is no way to roll back to it.
You shouldn’t run into any issues in iOS 12.1.3, however. The minor update simply fixed some issues related to HomePod, CarPlay, Messages, and the new iPad Pro lineup.
iOS 12.1.4 will soon be here with a fix for the massive FaceTime bug, which has forced Apple to temporarily disable Group FaceTime.