The sim card lets is what lets the iPad connect to 3G/4G. You do understand that without 3G/4G or Wifi there is no connection to anything, and no message sending.
With that said, however, the iPad only uses the sim card for internet access, it cannot send SMS on its own. It is not a phone. To send SMSs, the iPad needs to by linked to an iPhone, the instructions for how to do this, are in the link I provided above.
Here it is again:
Use Continuity to connect your iPhone, iPad, iPod touch, and Mac - Apple Support
Your iPhone, iPad, and iPod touch need to use iOS 8.1, and your Mac needs to use OS X Yosemite.
- Sign in to iMessage on your iPhone, your other iOS devices, and your Mac using the same Apple ID.
- On your iPhone: Go to Settings > Messages > Send & Receive > You Can Be Reached By, and add a check to both your phone number and email address. Then, go to Settings > Messages > Text Message Forwarding, and enable the device(s) you would like to forward messages to.
- Your Mac, iPad, or iPod touch will display a code. Enter this code on your iPhone to verify the SMS feature.