Setting throttle channel failsafe on FrSky Taranis + D4R-II

Move throttle to the "off" position.

Push trim on the throttle channel down as far as possible.
Alternately, increase servo rates on the throttle channel to maximum (i.e. 125%).

Move mode switch to the position for "autonomous" or "aided" mode.

Follow manufacturer's instructions for setting failsafe on the receiver. For some systems, you simply press a button. On others, failsafe is only set when the receiver is a bound to the transmitter.

After you're done binding, remember to restore throttle trim to normal / centered position and normal 100% servo rates. Otherwise, Ruby will always think that signal has been lost and will never give you control unless you happen to apply throttle.