Sim Not being read

I had an HTC Desire 626 and now I got an AT&T Samsung Galaxy s6 edge. The htc was with virgin mobile and the galaxy is with at&t. But my galaxy is now an unlocked phone so I wanted to use the old sim card from my htc. And whenever I put it in I can't get service and it says that my sim card isn't valid or something and tells me to restart my device to try and get it to connect to service and if the problem keeps persisting to call the support line. I called customer service but they told me they couldn't help me.