Hey there.
I was wandering if there is any module or something like that to 'connect' two Arduino Nano (or STM32) boards together via some radio frequency to send a small amount of data.
I have two Arduino Nano boards and two STM32 boards that are almost the same in size, but has a bit different specs (Flash/Ram - 32kb/2kb against 64kb/20kb) - project will depend on its size. I prefer Arduino Nano, but if I exceed 32kb of flash memory, I will have to choose STM board instead (just a bit more complexity in programming).
About my project:
I want to connect one Arduino board with LEDs (don't know exact amount yet). I would like to use Red/Green LEDs, but RGB LEDs are considered too, it doesn't matter to be honest.
Second arduino will be connected to some sensors (like obstacle for instance). Let's just say that they will be recording distance and write some values to an SD card at a set time, like every once in a minute. What I want to do is: that Arduino with LEDs would send a signal to another one with sensors. Sensor Arduino then would scan those values from SD card and if there is a change in value - I get RED LED to the LED Arduino, if not - Green.
Main Problems:
I live in a city, uptown to be more precise. There are some buildings, but not many and then massive forest. I want to put that Sensor Arduino in my summer cottage (battery powered) and LED Arduino into my apartment. Distance would be 10km/12km Straight/Road from those places. I know it doesn't sound much and I probably could use something like LoRa module. However, as I said - there are a few buildings and a massive forest in that way. So, my questions would be - is there anything that could go that far without LoS (Line-of-Sight)?
I don't care about time - it would only be checked once a day and anyways, I should receive that 'output' fairly quickly. I just power on my LED Arduino at home --> Signal sent --> Some waiting --> I receive LED colors --> Power down - that's it. I just want to monitor once a day that summer cottage without physically going there. If every LED is green - good, if one of them gets red - something's wrong and I need to go check.
Ask me anything, if You haven't understood something. And I'm sorry for my English (not my native language) and if I put my post in bad place.
Thank You in advance 
To those Legality Seekers out there in every post about 'Arduino 100km range':
- There are some radio frequency laws in my country, but only minor compared to USA or other BIG countries. I can't use only like 4 or 5 frequencies AND I'm far away from city center. I don't live in busy place and there is no one around that I could be disturbing. Almost everything is imported, so no factories, businesses or other important buildings are near me or in my country. I could even use a 2W or 10W booster and no one would say anything to me. Even my RC glider uses 900mhz Radio Link with four 3.5W boosters, powerful antennas and a ground station - no complaints, not even police (they've seen me flying few times).
- This 'device' will only be used once a day (before sleep):
a. To save battery power (should last a week or so)
b. To avoid huge workload of Sensor Arduino, that would have to always scan and scan those values.
So, keeping in mind that this would be used for very short amount of time and there isn't anyone I could possibly be disturbing - keep all that Legality BS to Yourselves and go bark somewhere else... Please...
Thank You too. 