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Here are a few things you could try:
Take the SIM card out of it's slot, and put it in again. Maybe that will help with your error.
If possible, try it on another device. If it doesn't work on a different device, the SIM card is faulty, and you need a new one.
Use a different SIM card on your iPad. If the other SIM doesn't work on your iPad, a hardware issue is causing this problem.
Turn on Airplane Mode in Settings - Airplane Mode, then wait for a few minutes and turn it off again.
Go to Settings - General - Reset, and reset network settings. You'll have to set up Wi-Fi and cellular data afterwards.
Also make sure your iPad is up to date. The latest software for the iPad Air is iOS 10.2.
Solving the problem with your SIM card will also be the solution for the greyed out setting.