The below guide has been created to reflect the fact that you actually CAN install apps manually using iTunes 12.7 onwards. It was previously reported that this wasn't able to be done after you upgraded past 12.6. You need to have already followed the guide Here for rolling back your app and getting version 4.1.12 (or older) it you're using this method to you can once again use the .dji.configs files, more detail on those HERE. Users with iPhone XS can therefore now use this method. Along with recent changes meaning you can no longer disable 2FA once you have enabled it, the guide has been re-written with new information and steps to still allow this to function. This has been manually verified by me (cs2000) on 22/12/2018
If you have an iPhone later than the X, your phone will refuse to talk to iTunes 12.6 as its too old. You should uninstall 12.6 and just grab the latest version from Apple before continuing as the images shown are from the new version of iTunes.
1. Connect iOS device to PC via USB.
2. Open iTunes.
3. Click on device icon:
4. Drag-and-drop an .ipa file to “On My Device” section on the left panel of iTunes:
5.If sync hasn't started automatically, click on “Sync” button and wait until synchronization is completed.
6. The app will then be installed.
You can now go and read DJI Configs Files for info on loading the .dji.configs to get FCC and boost power.