Original post by: Mr A ,
Have you already installed the iOS 12.1.3 update? if so then what your describing happens to a lot of apple users. this link http://osxdaily.com/2019/01/26/ios-12-1-3-cellular-data-problems-fix-iphone/ should help you fix the problem. Other than that have you tried rebooting it in airplane mode or resetting the network settings?