I think I figured it out. Short answer. DO NOT USE 2.4Ghz on your tablet or phone that is connected to the P3.
My Nexus 7 only has 2.4Ghz (No 5.8Ghz, no 4G). When I tried Screen Mirroring to my SmartTV I got the same exact results as when I was trying to to YouTube Live. I could not fly very far without losing HD signal.
So I tried using my Droid Turbo 2 phone which has 2.4Ghz and 5.8Ghz. I set it up to only use 5.8Ghz and then connected to my router's 5.8Ghz channel. My SmartTV is connected to my router via the 2.4Ghz channel, but because both are connected to the same router, it doesn't matter they can find and connect to each other.
I was able to Screen Mirror flawlessly using the 5.8Ghz channel on with my phone. No loss in distance or signal quality. The picture was pretty good on the TV for it being 720p.
However, like
sleagues, when I tried to YouTube Live stream with my Droid Turbo 2, I could get to Step 1, but even waiting much longer than 60 seconds it never went live. I believe this to be an issue with the phone and YouTube Live, not the P3 or the DJI Go app, as those worked with my Nexus 7.
YouTube Live is still in Beta, so my guess is that if it doesn't work for you, try another device. You can try 2.4Ghz to see if it works, but you won't be able to go very far. You will have to use 5.8Ghz or 4G (hope you have a big data plan).
p.s, I also noticed that if I turn off 2.4Ghz on my tablet before flying I can go maybe another 1000'. So basically always turn off 2.4Ghz when you fly for best results.
-Scott