SIM card homelessness

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. As such, a WiFi Only model fundamentally cannot directly access Cellular networkss

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 A2604, A2603, A2605 model, your iPad should support the eSim or have a SIM port.

In case you have a A2602, there is no cellular option, only Wi-Fi.

Check this article for more details on your iPad:

Identify your iPad model - Apple Support