Based on my experiences (I work for an ISP but we have a "sister" company that's an MSP that provides IT services to, among others, several K-12 schools in such rural areas), most of these schools either 1) already have available hardware (iPads, Chromebooks) due to existing 1:1 programs or 2) have decided to not (yet) pursue that (usually because of financial reasons).
In the former case, they already have hardware and don't want to buy even more. In the latter, they simply don't have the money for it (it'll be an even harder sell if it's not "general purpose" and can only be used with your stuff). Even if you manage to get it under $50 per student, that's still a lot of money that they could likely spend elsewhere.
Develop your applications and concentrate on targeting the most popular platforms (iOS and Android) and forget about trying to sell them hardware they can't (easily) reuse. Definitely forget about cellular connectivity and stick to Wi-Fi instead.