I've had a similar problem ever since I upgraded to iOS 12. My iPad Pro 9.7 is a Verizon model, and as pointed out above the embedded Apple SIM is disabled. However, I was using an external Apple SIM and was able to order data service just fine up until the time I upgraded to iOS 12. After the upgrade the options to purchase data services disappeared from the cellular settings. I opened a case w/ Apple Support and eventually had an appointment with the local Genius Bar, but they couldn't solve the issue. They had me reset the network settings, eventually reset the device and restored the factory settings, etc...but no luck. They even popped a brand new Apple SIM into the nano SIM slot, thinking that perhaps my existing SIM had gone bad, but the device still did not recognize it. Then finally we popped a carrier SIM into the slot and it recognized it just fine. So...it appears that there is an issue with iOS 12 somehow not recognizing the Apple SIM properly.