The oval “blob” is bug introduced in iOS12 - and seemingly affects only the older Cellular models of iPad. When it occurs, the blob obscures the Cellular signal-strength indicator, regardless of having a SIM being present in the SIM tray.
Thus far, the problem has not been resolved in any of iOS12 point-releases - athough, with installation of an update, the blob has been seen to temporarily “clear” but return some time (often many days) later. The specific trigger has not been identified.
Thus far, the problem appears to be cosmetic only.
It is suggested that you file a report via the Apple feedback channel:
https://www.apple.com/feedback/ipad.html
I hope this information is helpful - if only to put your mind at rest in knowing that this is both a known issue and that you’re not alone.