With a tethered boot you HAVE to be able to connect the ATV to the PC/Mac via USB to complete the boot process, and then unplug the USB and plug in the HDMI WITHOUT ever disconnecting the power. You therefore either need the ATV and PC/Mac to be close to each other or to be using a laptop.
It is likely that an untethered boot will appear at some point which will remove the need to keep the power connected all the time, but the timescale for that is unknown.
Thats my issue. I have to unplug the apple TV from power at my desk then run across the house to reconnect the appletv to my power source where the TV is.
The tethered reboot only works if you never loose power.
Pulling out the laptop. Will let you know. Thank you.