Can I add cellular service on an iPad that only has wifi?

You cannot add cellular capability to the iPad. You either buy a cellular plus Wi-Fi iPad or a Wi-Fi only iPad. but you can purchase a Mi-Fi device that will connect to a cellular data network and connect to your iPad via Wi-Fi.

You can't add it to the 'guts' of the iPad. It's a sealed system.

What you can do is go for a device like a Mifi. It's a battery operated portable wifi hotspot that you can then connect your ipad to.

I believe that every major provider has their version of the device so look for hte provider that offers you the best options and go from there.

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