Cellular iPad-Air text message configuration
You can't use SMS messaging on an iOS device (or any device for that matter) that doesn't connect to the GSM network.

SMS is part of the GSM services network; the "telephone" functionality of your device.  It's the network that allows you to place a call to all phones including land lines.  The iPad doesn't connect to the GSM network.

iMessage requires a data connection (WiFi or cellular data like LTE) because they are sent and received through a server over the TCP/IP protocol.