When I run my app with the simulator or on a physical device (iPhone 10s, iPad 7), it works fine. So I send the archive to the App Store and they reject it because all they get is a black screen.
Two questions:
Apple's reply should include the device and iOS version they used to test. Try running it in the same conditions.
Please tell if you find the solution. I have pretty much the same problem with my Unity iOS app.
In my case, there was a problem with how Unity made the iOS build that would cause the load time at the beginning to be too long (it would never load and then the process would be shut down for taking too long). This problem did not show up during testing unless I turned off the debug executable. If you are using Unity try this solution. If you are not using Unity, the solution may still be relevant.
I solved this problem by creating a new project and copying the files from the failing project to the new one. The app from the new project was accepted by the App Store.