You could always try XBee. The XBee Pro modules are a bit expensive (~$50 each), but they can reach up to 15 miles or more, albeit at fairly low baud rates. The Pro-900 XSC ( https://www.sparkfun.com/products/11634 ) can stretch up to 28 miles with just its wire antenna. There's also LoRaWAN, which is even longer range but still low bandwidth and $$.
