I do not know what you were doing or think you were doing, but no iPad has ever been able to natively send or receive SMS or MMS messages. And no carrier offering iPad data plans offers texting because the device itself is not capable of doing it.
SMS texts have an obligatory requirement for a cellular telephone voice channel. They do not and cannot be sent over a cellular data connection or any other form of internet connection. They are sent in a small unused portion of the cellular voice channel bandwidth.
Since no iPad can make a cellular telephone voice connection (and no iPad has ever been able to do so) as it does not have the hardware to do that, no iPad can send an SMS text by itself. And no iPad ever has been able to do so.
When Apple developed the continuity feature in iOS and OS X (introduced with iOS 8 and OS X 10.10) , it then became possible to pair an iPhone with an iPad. The iPad then uses the iPhone to actually send and receive SMS texts. But without an iPhone, an iPad has no such capability and never has.
An iPad by itself is limited to internet messaging services only, such as Apple’s free iMeassage Service. You can use email and carrier’s SMS email gateways to send people SMS texts as email as well (e.g. for an AT&T customer, emailing the message to there 10-digit-telephone-number@txt.att.net).