Phill_Phan, yes, both cell service and cell data were lost because the phone would not recongnize the SIM card. No ICCID line would display for the SIM card under Settings:General:About.
The fix from havok565 has allowed me to restore service, and I have called both Apple and ATT tech support to alert them to this so they stop telling people to erase their phones!!!
I have left Find My iPhone off for now. I did have one call dropped and experienced a subsequent loss of service after using the workaround the first time. I have no idea if leaving Find My iPhone off will prevent a recurrence, but it did prove to be true that one can restore service again using this method if it goes down again.
Remember that when I first upgraded to 11.0.3 it took almost 18 hours beforre my cell service died. At least Apple’s software engineers now have a hint at what to look for!!
The other take-home message is don’t let people talk you into erasing your phone if you have good reason to believe that you have not changed anything, dropped it, etc., to warrant such extreme action. The problem was clearly the with software upgrade, and I am extremely grateful to havok565 for the discovery of the woerkaround!!