To me, a legitimate answer would be to use Arduino to switch a very bright light and send morse code which a human could read miles away. I have a feeling that won't work for you although it meets this spec completely:
I was wondering how can we send data from an Arduino in real-time over long distances (such as a mile -- few miles).
WiFi connection does not work for this, because it is mostly used for shorter distances.
How can I connect an Arduino wirelessly over long distances?
The small LoRa tracker PCBs I sell can be used for remote battery powered sensors;
Used as balloon trackers I have had them working at 270km on only 10mW, but thats at 1500bps, drop the data rate to 100bps and the distance should go up to circa 750km.