I experience a „no cellular network“ issue with both of my devices (IPad Pro 10.5 / iPhoneX) on IOS12b2. If the cellular network ist not showing up, the issue seems to be the missing -enter sim code- dialog. You are simply not asked to enter your 4 digit code.
The remedy, which helps for me is, remove the SIM card and insert it again, then the the dialog apears. Enter the code and go to -Settings-. Now you see the settings area for the mobile network again. I deactivated the code for the time being.
Hope this helps.
Chris