Hey, I have a small project. I want to make a small device that can communicate with other devices. If a button on one of the maybe 4-5 devices is pressed, it should send a signal to all other devices, that that button has been pressed on, for example: Device 2).
The enviroment is outside with obstacles, like tress and some walls. It should have a range of about 300 meters.
What technology should i use here?
And if working with radio frequencies, it should be legal in europe (france, germany and austria).
Investigate what is available commercially. That will tell you what is legal in your country.
LoRa
This seems to be the key part of your project, so develop and test the communications part before doing any other part. Do not leave it until the last thing!
You need to do pratical testing in the sort of area where you want to use a project.
There can be no precise prediction of how far a particular type of radio module will get, trees vary, walls vary.
Radio modules such as HC-12, RFM69 etc, migh well be OK for your application, but typically LoRa modules will go 10 times further the distance and may work when other radios stop working dues to trees an walls etc.
Get testing.
You don't write anything regarding the needed airtime - how long and how often do you expect that the button gets pressed in one hour?
LoRa on 868MHz might be an option (but you have to take care about the needed airtime).
I would also do some tests with ESP-Now. 300m without line of sight might be an issue, but when you have deployed other devices they could relay the message like in a Mesh network.