On version 4.1.9 , 4.1.8 and 4.1.3 any region windows don't show up on my app.
But I found 2 tips:
1. very important is to turn ON controller and app first, finaly Mavic last.
2. in Menu "Image Transmission setting" We can check how many channels is active 12 or 13, in some cases 14.
Simple turn in Custom Frequency selection, then move "square" to the end right. If You see 2465,5 MHz then this is 12 channels = FCC, if can move it more for bigest frequency like 2476,5MHz or 2481,5MHz then this is NOT FCC. Probably I'm right. ;)
In my case if I turn ON Mavic first then app it will go to CE and have 13 channels, then I must turn OFF and again ON mavic and app back to 12 channels.
Waiting for range test from You.