iphone 5 no service/searching..

So here's my situation:

I bought an unlock code online yesterday, so I went through the trouble of backing up and restoring my phone. During the restore, it also updated my firmware to 6.1.4. Then after restoring it, I got the message that says "Congrats, you have unlocked your iphone(or something similiar to that)". I used the last back up I created for my phone to set everything back up. After I did that, I got a little curious and also went to Settings/General/Reset and pressed on "Erase All Content and Settings." And then I proceed with setting up my phone but noticing that it kept saying No Service or Searching.. I don't know if I had service before I pressed reset under 'General. I've had contact AT&T tech support, and they had lead me through with all the solutions he knew about. However, nothing works. So he recommend me to switch into a new sim card and see if it would help out with my problem. I haven't had time to go to the store and pick it up yet. I'm just wondering if anyone has an idea how to solve this issue of mines.

Thank You!