How to enable 3G for an Indian iPad on Airtel?
I found the answer myself and I thought I should put it out there for another hapless (recent iOS) convert drawn like a moth to the flame that is these beautiful yet quite flawed iOS devices, so that they may continue make the most of their devices in good network coverage or poor.

So, how do you get an iPad Air 2 LTE (running iOS 8.x) to the ability to use 3G on Airtel India in Bangalore, Karnataka?

Without much further ado, I present to you the answer .... 

... do a factory restore using an IPSW file.

Its really simple to do this. I'll give you the general outline.


Backup your iPad or iPhone using iTunes or iCloud. You should already know how to do this.
Download the IPSW file for your device onto your computer. This link is a good place to start. There are other websites which provide the same service.
Connect your iOS device to iTunes and select restore device. This link has more details.
Keep your device connected to the computer throughout the restore process.
When the device has booted up after the reset, iTunes will ask you to check for the carrier settings check. Allow it do so... and you will have 3G working on your iOS device.


I hope this helps someone else.