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Do you have a WiFi+Cellular model iPad, or WiFi Only?

If you have a Cellular model iPad:

Set up cellular data service on your Wi-Fi + Cellular model iPad - Apple Support

WiFi Only models do not have any Cellular capabilities - and hardware cannot be retrofitted. It is not possible to change or modify the internal hardware components of an iPad. 

Whilst a WiFi Only iPad cannot be transformed into a WiFi+Cellular model, you can use the hotspot feature of a mobile phone - or a Cellular Broadband Router (often known as a MiFi Router) - to access Cellular networks.

How to set up a Personal Hotspot on your iPhone or iPad – Apple Support