If you want to use an Airtel Nigeria SIM card on your Apple 3G iPad, you will have to buy the Airtel micro SIM from an Airtel office, register the SIM and wait for it to be activated. Once activated, you can insert it into the iPad for you to start surfing the internet with your iPad but before that can be possible, you will have to configure your iPad with Airtel internet settings (Airtel APN settings).
How To Configure Airtel APN Settings on Apple iPad
==> Tap the Settings icon on your iPad screen
==> Tap "Mobile data" or "Cellular data". You should now see "APN settings" as seen in the picture below
==> Tap the "APN Settings" and input the settings below:
==> You can now press the "Home button" to exit the "settings" page.
==> Launch your safari browser and you should now be able to browse the internet with your iPad using an Airtel Nigeria SIM.
NOTE: You must have recharged and subscribed to Airtel Internet bundles before you can browse the net. You can easily do this by inserting your micro SIM into your phone before inserting it into your iPad.
You will also have to follow the same steps if you want to use your Etisalat SIM, GloMobile SIM or MTN SIM on your iPad, though you have to configure each network with its own APN settings.
I hope this works.
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