Sim card

You cannot add Cellular capability to a WiFi Only model of iPad - however, you can access Cellular service from a WiFi hotspot.

There are two main ways to provide a WiFi hotspot:

1) A mobile phone - that can provide a WiFi Hotspot. An iPhone is ideal, as the hotspot connection can be initiated directly from the iPad. Other brands of mobile phone will usually suffice, but the hotspot connection must be manually enabled each time that you wish to use it. It follows that the Cellular contract must allow connection via the WiFi hotspot (otherwise known as tethering).

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

2) A flexible alternative is to purchase a mobile broadband router - often known as MiFi - and an appropriate cellular airtime contract from your chosen Cellular provider.

A third option is to consider replacement of your WiFi Only iPad with a Cellular+WiFi model. Owning the latter doesn’t commit you to needing to use the Cellular connection, but provides flexibility should you ever need to do so. The Cellular models of iPad also include full GPS capabilities- this feature being entirely absent from WiFi Only models.

I hope this information, in exploring your available options, provides some helpful food for thought.

You cannot "add" cellular to a wi-fi only iPad. You have to purchase an iPad that already is equipped with cellular capabilities. There's more to it than just adding a SIM card.