I use Spektrum with Powerbox Mercury.
The Spektrum Xicoy Telemetry needs power at startup in order for the Spektrum Xbus to find it and initialize it. The problem is with a RX controlled switch it needs power from the RX to complete the circuit and power the xicoy telemetry, this takes too long and the Xbus will ignore the xicoy on startup.

The easiest way is to run a separate power lead to the JR plug on the Xicoy with a small switch. After you power up your model and everything is active you can switch off this power lead with the switch. You RX controlled switch will now cycle the power off to reset fuel.

This is one way..there can be others...