So, in the end the solution to go back to fcc mode is this:
1 - Uninstal Dji Go app from your mobile, also delete DJI folder and the two hidden folders .HereMaps and AMAP
2 - Install Fake Gps, launch and point it to some place in the USA (you need internet connection to see the map) NOTE: Be sure tu only use gps for positioning services!
3 - Install Dji go 3.1.23 or any recent version you want that actually switches from fcc to ce. Launch it and log in then exit app and shut internet connection, better delete any wifi nearby.
4 - Turn receiver on and connect mobile and receiver.
5 - Turn on aircraft and let it bind to receiver
6 - Launch Dji go and enter camera view, wait for the wlan window to show and click yes
7 - Unistall dji go and delete all the folders as in point 1
8 - Install dji go app 3.1.8 (here is a link
Free Download APK File of the DJI GO v3.1.8 for Android - APK4Fun )
9 - launch dji go and log in
Now you can turn fake gps off and you're ready to fly as you always did!
I think there's not realy need of fake gps when you install 3.1.8 version, but I haven't check so this way I'm sure it works!
FIRMWARE version: I used 1.3.20 just because I had this on, but I don't think it will affect the result, I'll check this too later