Unfortunately AT&T requires you to use their "AT&T Messages" app for your tablet. This is not a Samsung limitation or requirement, it is imposed completely by AT&T and forced onto the Samsung device.
Another solution that might work is to use the Google messages app on your phone instead of the Samsung messages app. Then in Chrome on your tablet you can go to messages.google.com and link it with your phone. Then you can see all your messages and respond as needed. I do this on my computer all the time but haven't tried it on a tablet.