iPad

Cellular capability cannot be added after purchase - a WiFi Only iPad is always a WiFi Only iPad.

You have two alternatives to buying a new iPad.

1) If you have an iPhone (or other mobile phone), subject to support by your chosen Cellular carrier, you can use the WiFi hotspot feature.

2) You could use a cellular broadband router (sometimes known as a MiFi router) to create a WiFi hotspot. You just need a router (often sold by cellular carriers along with an airtime contract) and many retailers (e.g., Amazon).

iPads are sold as Wi-Fi only or Wi-Fi + Cellular. If you bought the Wi-Fi only model there is no way to add cellular to it. You would need to buy one with cellular capability.