You may refer to "Important Notes" section in About > FPV Camera > Features for the basics as below.
"When you switch back-and-forth between DJI VISION app and FPV Camera (or other DJI SDK apps), you may need to completely quit non-used apps (by BACK button all the way out) to avoid locking up FPV connection in the background. Otherwise, you may see "Phantom Connection Broken" and "Video Signal Lost" messages shown on FPV Camera screen."
- I read this but I only opened the booster app, run the boost feature then backed out and opened the DJI app. I never started your FPV camera app, yet still got no video feed or connection