1. Download Jepp FD v2.7.4 from AppStore
Note: the latest version is probably not v2.7.4 and you MUST downgrade to v2.7.4 for this to work, consider downloading an old version of iTunes and use it in combination with Fiddler to grab the old ipa for Jepp FD. Again, you must be on v2.7.4 which should support anything from iOS 8 - 13.
2. Search in web archive for an outdated link in russian aviation forums that contains the data files for Jepp FD. It should be a yandex disk link hosting around ~2GB of files (and hasn't expired as of July 2020). For copyright reasons I cannot post the link here but you should be able to find it. You have to download the entirety of them for it to work.
3. Go to a) or b) depending on your device
a) If your device is jailbroken - copy the data files into /var/mobile/containers/Data/Application/[folder for Jepp FD]/
b) If not, backup your device first, edit the backup with iMazing or equivalent, then restore it back to the device
4. ???
5. Profit
That's it guys - hope y'all will succeed in getting this to work.