After a particularly serious bug in iOS 12.1.3 paralyzed Facetime for groups, Apple released iOS 12.1.4 on February 7. This update is now said to cause massive problems when using the O2 eSim on iPhone and Apple Watch.
Apparently, O2 customers can use the latest iOS software update can no longer use their eSim correctly in the Apple Watch and the new iPhone models. A network operator update is to blame. An O2 employee has now issued an explanation for the problem. Accordingly, Apple has updated the network operator settings at O2 at the same time as iOS 12.1.4. The current version is said to be o2-de 35.2. Users who downloaded this version before the most recent iOS update are therefore not affected. All O2 customers who downloaded iOS 12.1.4 before the new network operator update may have a limited mobile connection. According to the O2 spokesperson, they are already in contact with Apple Germany to fix the error. The Australian provider Telstra also appears to have the same problem.
This is how the problem can be solved
It may take some time before a corresponding update is released. If you don't want to wait that long, you can go back to iOS 12.1.3 using iTunes or follow O2's recommendation and install the public beta of iOS 12.2 on your iPhone.