Can I add cellular to my iPad air2

Yes if you bought a iPad Wi-Fi + Cellular model.

You cannot add later.

Not unless you bought an iPad that has cellular capability. Identify your iPad model - Apple Support

-AJ

You cannot alter the hardware of a wifi-only iPad. You can use a smartphone hotspot or a mifi device to connect over cellular networks though.

Hi

If your ipad has a plastic strip at the top on the back then its will use a data simcard if its just plain at the back

Buy a mifi dongle contact or pay & go you can run up to 10 devices at once.

Cheers

Brian