"Okay, Google. Turn on Shield" (or other nickname) should work. There are some apps that don't work, but I've found for YouTube TV, I can say "Okay, Google. Watch ESPN." and it turns everything on and flips to ESPN (sometimes I get an error, but it still turns on).

Alternatively, you could try connecting your phone to the Shield TV via the Android TV Remote Control App. If CEC is working, it may wake the shield.

If it isn't working still, it is worth double checking the settings on all of the devices involved. To get mine to work, I had to make sure my Shield was correctly listed in the Home app and give it a nickname (gave it a different name, but I still ask to watch on "Shield"). For whatever reason, this wasn't an instant update and I had to wait a day or so for it to fully work. I also had to make sure CEC was enabled on the Shield for both "on" and "off" (also did for volume). I think my TV had CEC listed in multiple places, and I also had to find the CEC settings in my AVR.

Make sure you are connected via the HDMI ARC of your TV and CEC is enabled on your TV as well!

You can't change the name of your device unless it's a Chromecast stick.

So try saying "OK google, turn on my whatever the name of your device is "

I put TV as the name of my Chromecast. So when I say "OK Google, turn on tv" it works.

In Google assistant settings on your TV go to "TVs and speakers". Assign a room to your shield tv.

I can say "hey Google, turn on/off the living room tv"

Also make sure on your phone that in the Google app settings > assistant, that your shield is showing and set up correctly!

The only 100 % guaranteed way I can turn on my shield with my phone is chrome cast to it. Then it wakes up and you cam use the android remote

I just hit a button on the remote and it turns it on.