The Onkyo app is very convenient for controlling your receiver and queuing up songs from a media server but one problem is that if the receiver is turned off, you can't turn it on from inside the app. Or can you? The trick is that you need to put the receiver into Standby mode. The way to enable Standby mode is to turn on the HDMI pass-through function.
Press the Setup button
Navigate to [Hardware Setup] > [HDMI] > [HDMI Through]
Set to Auto
While in Standby mode, you can connect using the app and power up the receiver.