Ah. Thanks for sharing! I experienced this yesterday. My carrier thought it was the SIM, so they replaced it. I purchased a new iPhone X last week, and throughout the day yesterday, I was getting a message no cellular data. In my iPhone settings for cellular, it showed no SIM. I reboot the phone, and after 1 minute I get a cellular connection that lasts 5-10 minutes. I think you're right about iOS 12.1.2 update because it was two day after purchase before I updated the phone iOS 12.1.2.
Soon after updating to ios 12.1.2 my data entirely stopped working via wifi or through normal isp. I could receive calls and texts and would even get some push notifications, but could not access any internet, email, and most apps became useless. My text app was acting weird as well, I tried everything including the steps above and nothing worked. Then I realized one thing I had not done...remove possible conflicting apps. about three weeks prior to updating, I had installed Phone Guardian Mobile Security app. It monitors public wifis to protect your phone privacy using VPN technology to protect browsing activity and encrypting all http traffic on networks your don't trust. At any rate, my experience is security apps often conflict with updates and other apps. I removed the security app from my phone and immediately data resumed. two days worth of emails and other alerts suddenly appeared. fixed. just saying, if you have a security app installed and are experiencing data issues after 12.1.2 update, then consider removing it to see if anything changes.