Take a look at the HopeRF transmitters. The RFM69 range works with Particle, see the library info discussion. You can select versions in both 433 MHz and 915 MHz (however sounds like you have to go with 433). The high power “H” +20 dBm version will give you way beyond 50m range (under the right conditions yadda yadda).
However, I’m not sure how you will sniff the required transmission for activating each device (you can purchase tools for this). And then making sure you can control the transmission satisfactorily to produce the right signal. But would love to hear about your progress!
PS: If you’re testing it with dumb devices you’ll definitely want to switch off the encryption capabilities in the library.