I think there is a solution to put the Phantom 3SE back to FCC mode. I started again with the first firmware, version 01.01.0000. Then I updated to version 1.03.0020.
The remarkable thing is that my P3SE remains in FCC mode, even when using the latest version of the app (v3.1.23).
The remarkable thing is that my P3SE remains in FCC mode, even when using the latest version of the app (v3.1.23).