I hope I can shed some light on to this subject. I got my quad in December and quickly updated the camera firmware to 1.1.9 Everything worked great. I put it away for a couple of weeks and went out to fly (cold weather grounded me). At that time, I had the dreaded "black screen". Everything worked except for the live video feed. I could control the camera pitch, take photos and videos, see the telemetry...everything was normal except I couldn't see the live feed. Grrr. I took it up and fly it blindly and the videos and photos were normal. They looked great. However, it's nerve wracking to be flying blindly.
So, I hit the internet and looked for a solution. It seems there are hundreds of people with the same problem and nobody really had an answer. I refused to believe my wifi card had mysteriously crapped out, sitting in it's case. It just had to be either a software problem or some other problem. I researched and researched. I even found out that the p2v+ has the wifi card in the main body while the p2v has the wifi card in the camera unit. I was thisclose to buying a new camera unit (fc200) just to solve this problem. Thank God I didn't! After reading this forum and many others...and trying lots of variations, I've finally figured it out. Well, maybe not everyone's problems...but, I figured out mine. And, maybe this will work for you too. I had downloaded the firmware update back in December and it worked then. I had downloaded it 4 other times since then, and I used the latest version every time I downloaded (not sure if that was a detriment or had no bearing whatsoever). Regardless...I decided to go back to the original firmware download I did, back in December. I dug around until I found the micro SD card that came with it and used that, EXACTLY the way the DJI video suggested. I formatted it using the Vision app in the quad. Next, I unzipped the original firmware download and copied the bin file to the micro SD card. I then took the card and inserted it into the camera (with the camera in the 'camera on' position and the quad powered off). I then powered up the quad and let it install the firmware. Once I had a green light, I powered off the quad. I then powered up the quad and confirmed I had a green light on the camera. I powered off the quad and removed the card. I'm using the same card that came with it, only because I'm paranoid another card might screw it up again.
With the camera loaded with the firmware, I've come up with the following sequence and it works for me.
1. Turn off the data on your phone (disable 4G)
2. Turn on airplane mode
3. Turn off wifi
4. Power on transmitter
5. Power on wifi extender
6. Power on quad(battery pack)
7. Turn on the camera from the "off" position to the "wifi on" position
8. Turn on wifi on phone
9. Find Phantom on the wifi list and connect to the Phantom
10. Turn on Vision app on phone.
You should now be able to click on the camera and see a live video feed from your camera.
I hope this helps.