New iPad ninth generation

No.

iPad has internal hardware that is buikt-into cellular data model iPads.

It cannot simply be ”added later”

If you needed both a WiFi and cellular data service iPad, then that is what you should have purchased.

That stated,

If you own an iPhone or any other brand of smart phone that has WiFi hotspot feature capability, you CAN connect that iPad to the WiFi hot spot feature of a smart phone and your iPad would have cellular connectivity through a WiFi connection that way.

Otherwise a WiFi ONLY iPad model contains NO internal cellular data service hardware components for any cellular service use.

It CANNOT BE added later!

Sorry.

No. You can't add cellular to iPad after the time of purchase. Wi-Fi + Cellular models of iPad have additional networking that allows them to connect to LTE.

If you want to purchase a cellular plan at a later date for your iPad, you should return your iPad or cancel your order, then purchase an iPad with Wi-Fi + Cellular. You don't need to purchase a data plan for your iPad right away.

iPad + Cellular - Apple

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