At present, Apple has not given a solution for this issue, even update to iOS12.1.2. You can wait for Apple iOS 12.1.3. Before it, You can solve this issue by restore iPhone, of course, you will be lost all your data. If there is no IMEI after restore, then check the Baseband rail, otherwise, change the iPhone 8 Radio IC with a new one. More about iOS12.1.1 problems and solutions you can check Apple iOS12.1.1 Problems & Solutions – All You Should Know.
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REWA is a world leading electronics repair business solutions provider who was founded in 2008 in HongKong. We are committed to delivering one-stop services covering Sourcing Solution, Technical Support Solution as well as Recycle & Resell Solution.