DJI is the world leader in aerial photography systems. At the DJI official website, learn about consumer products like Mavic drones and Osmo, DJI OM 4 handheld stabilizers & cameras, professional tools like Ronin and Inspire, enterprise platforms like Matrice 300 RTK, and agriculture solutions...
You can Google "dji fc40 apk".
In the search results find "apk lean" website, probably the top of the list of search results.
They have it for download. Dji no longer offers the app. I think Litchi also has an app for it but I'm not sure.
I hope this fixes your dilemma.