Also google uses a play protect system that stops apps being installed that havent come from the playstore. this can be turned off in playstore settings
Ok.
So, here's what happened.
DJI over on their forum gave me a link so that I could download the apk onto my computer.
I then pushed that one over to the tablet via USB.
Then, using ADB, I remotely forced the tablet to install the apk...
I tried searching from the settings search for install unknown. No results found.
Drilling down into install unknown apps, it says no apps. The hamburger menu only says 'show system'.
There does not seem to be a way to add an app to that...