What version do I need for SMS to work?

SMS has never worked with any iPad (wi-fi or wi-fi + cellular). SMS requires a cell phone with a text messaging plan. See: About iMessage and SMS/MMS - Apple Support

It is possible to use Continuity to get iPhone SMS messages to an iPad. See: How to forward SMS/MMS text messages from your iPhone to your iPad, iPod touch or Mac - Apple Support

No iPad in the history of iPads, whether WiFi only or WiFi & cellular, has ever natively supported SMS nor MMS messaging.

An iPad can only use your carriers SMS/MMS service when paired with an iPhone as the iPhone actually sends and receives those over its cellular telephone voice channel connection.

By itself, an iPad is limited to internet based messaging services - iMessage, Skype, Google Hangouts, WhatsApp web, etc.

See the instructions in the following: How to forward SMS/MMS text messages from your iPhone to your iPad, iPod touch or Mac - Apple Support

Check the message settings on the iPhone to make sure it is still set to forward texts to that iPad. See the links in Foxfifth’s post.

Hello,

Firstly you cannot revert to a previous version of iPad’s system software. Secondly Apple has not disabled sending/receiving cellular text messages on iPad as long as Text Message Forwarding is enabled on the accompanied iPhone.