I had the same issue, but adopting the safety approach of turning the transmitter on first solves it.
I also programmed my receiver to output a immediate output failsafe values of 1500pwm on ail, ele, rud and 900pwm on thr (above radio failsafe) when first powered up with or without the tx switched on. However, it also has a setting to stop sending pwm signals on loss of transmitter contact so thr failsafe still works when flying.