rEceived May 20. Brand new P3 Profssional. Went through all the setup proceedures, connected my iPhone 6 plus with usb cable. App shows "disconnected" P3 lights up correctly. Battery full charge. RC full charge. Lights up correctly. Phone full charge. Launch DJI Pilot app. Shows "Disconnected". What now?

Try with another USB cable then another device.

The issue is  faulty USB port A .
I had same problem and I manage to change the broken usb port ( plastic inside was broken )  with new one ( 1 usd) .
And voila now I m back in bussines...... every time I connect with no problem IOS or Android.