Cellular Plan

All iPad models are available in two fundamental variants - these being WiFi Only and WiFi+Cellular.

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

For WiFi+Cellular models, you can easily add Cellular by adding DataSIM from your chosen Cellular Carrier, or for recent models that incorporate eSIM, you can add a Cellular plan directly from iPad settings - as described here:

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

You have to have an iPad that has cellular capabilities (depends on what you bought). If you do, you need to call a provider who will offer you a data plan. There is no set plan or cost - it is whatever the provider wants to offer.