I've just spent 4 hours getting this to work with my iPhone so sharing my experience in case it helps anyone else. I should point out that I'm in New Zealand so we're allowed to use FCC legally here but DJI seem to think we're part of Australia so force us onto CE.
I hadn't seen the video above so it would have made this easier!
The .DJI.configs method did not work for me (running latest Spark firmware) so I used an old Android phone with a modified DJIGo 4.1.3 APK file using the guide here
DJI Dashboard - How To tips and tricks *MAVIC* - RC Groups
Making that file wasn't that simple but once done I connected to my Spark and it prompted me to change the wireless settings. I swapped to my iPhone and clicked Cancel when it asked me to change the settings back.
Just did a test flight and got over 800m before I turned back and called it a success, previously I was getting video break up around 120m and signal loss around 150m so this is a huge improvement.
The only things I'll need to watch out for now are:
1) Don't click OK to change the wireless settings (although if I do then I can swap them back with the android)
2) Don't update Spark FW in case they fix this
3) Don't update the iOS app in case the force the setting change
