Apple has officially stopped signing signed iOS 12.1.1 beta 3 version, meaning that you can no longer downgrade in order to use your devices with Unc0ver jailbreak.
It is known that Apple’s new move is to prevent users from downgrading this version to jailbreak. That is, if you have not downgraded your iPhone to iOS 12.1.1 beta 3 or if it is not running iOS 12.1.1 – iOS 12.1.2, there is no way you can unlock it with Unc0ver. If for some reason, you can’t lower your iPhone to iOS 12.1.1 beta 3, you’re out of luck now.
It is known that the current version of iOS 12.1.3 is not completely jailbroken. Recent exploit codes like voucher_swap, v1ntex, v3ntex, and machswap are only compatible with iOS versions from iOS 11.0-12.1.2. If you are using iOS 12.0-12.1.2, you should keep your version the same if you want to maintain the jailbreak for your device.