This question already has an answer here:
I have an iPad Air 16gb I bought off a popular classifieds website. It was advertised as WiFi only, and in the settings there is only WiFi. But when I look up the serial number on Apple's site, it says it's WiFi + Cellular. I've corroborated this on several other Apple serial lookup sites. Am I missing something? Is there something I have to activate to get the cellular settings to appear?