From my past experience, Azure can get expensive quickly. It was on the order of $50/month to host an app that interacted with a wireless lock. It may have been able to do cheaper through Azure, but I found it slightly confusing when I used it in late 2017. I dont know about any solutions for low power cellular gateway hardware. A non cellular option might be LoRa radios, expecially if “The Things Network”, TTN, is in your area.