I don't think there is wrong with the app...usually the android version which not compatible....
There is 2 metode to change ce mode to fcc,
1st. Just with fake gps.....but you have use handphone with older android version (version 8)
2nd. You can use any version on android but tou have...
I can change to fcc mode for my mavic air 1, its doesn't matter if you use the latest dji go 4, and also newest android phone....
(I use samsung M51, and latest dji go 4)