Seems like perhaps the pin number 18 isn’t correct — in other words, it’s not matching the actual pin you are using. Clearly the firmware is able to send the signal out to pin 15. Maybe you should double check the wiring, and double check the GPIO number — this is the only possible issue I can think of.
In the worst case, you can probably just use GPIO 15 (the pin that’s connected to relay). The relay will make a noise, but the RF transmitter should still work along with it.