Are iPads carrier-locked?

I'm asking this because I'm planning to buy an iPad Air or if I can hold out for a few months, the next generation, and I'm planning to buy a refurbished one with LTE. All the available refurbished LTE iPad Airs are "from AT&T", and I'm worried that it may be locked because I'm not from the US (I'll only be shipping it from the US because it's ridiculously overpriced here) and obviously, I will not use AT&T's network. I know that LTE iPad Airs have all the LTE bands but I'm worried that my SIM card might not work. Do any of you have experiences with this kind of situation?