This happened to me intermittently on my 64gb AT&T version. I had to reset network settings or turn it off/on to get it to pick up again. This began straight out of the box. I called AT&T who said all they could do was replace the sim. I replaced the sim twice and it still happened. Went to the genius bar and they offered to replace it without any fuss.
They didn't have one in stock so they had to order one. I just got it a few days ago and no problems now with the cellular service on this iPad. Luckily the screen is great even seems brighter than the old one which seemed dim and no other issues with heat or anything else.
I'd return it while you are in the 14 day period. If you get a dud you can always return it again.