Removing public beta failing
you can get a recovery iOS on the development page:

https://developer.apple.com/download/

just sign in with your appleID download the recovery image and follow this introductions:

https://wccftech.com/downgrade-ios-114-to-ios-113/

the tutorial is for iOS 11 but it's the same for iOS 12.

Please make a backup before doing this!

Edit:

at https://developer.apple.com/download/ please log in with your appleID. after that you'll see a download list:



click on see all at the entry iOS Restore Images.
You'll see another list:



Download the restore Image for your iPhone.