Carrier unlock iPad

Ok, I bought a used iPad 2 several years ago off of Amazon.com. I've been using it wifi only because I didn't want to pay for additional data from my mobile carrier and that's been fine, but recently they offered a cheap data deal that I accepted and got a SIM card for. I've got everything set up but the iPad will not connect to my carrier, and I can't figure out any reason why it doesn't save for the possibility that it may still be locked to whatever carrier the device was originally sold through. There's no carrier on the back like they used to etch sometimes ("ATT iPhone" or whatever) so I don't have a clue how to go about getting it unlocked to work on my current carrier, if that's even the problem.

Any ideas?