iPad Pro

If you have a WiFi+Cellular model iPad, you can activate cellular service by installing a DataSIM that you will obtain from a Cellular Carrier of your choice, or for supported models, by activating a Cellular plan using the iPad's built-in eSIM:

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

If your iPad does not have a SIM slot, then you have a WiFi Only model of iPad. 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

If you have a cellular model iPad, this applies.

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

If you did not purchase a cellular model iPad, it cannot ever use cellular data.