Have you gone into Android Settings and allowed it to install apps that are not in the Playstore?
The Yuneec Pilot app was designed for an earlier version of Android (v5.0) and does not function well with newer versions beyond Android v7.0. You can try running an app that emulates earlier versions of Android to allow your present phone to work with Yuneec Pilot.
Another option is to try using an older iOS phone that runs iOS 10 or greater. The Yuneec Pilot app actually a bit better on iPhone/iPad.
I have used both iOS and Android versions of the app.
If you opt for an older compatible phone of either type of OS, you do not need to have cellular service. It is actually better if cellular is not active since the phone is being used as part of the Remote Control for the aircraft.
When you get the iPhone, open "APP Store", search for "Yuneec Pilot", and install it.
I just did this on an older iPhone SE with IOS 18.1.1., and it seems to have installed normally. It also worked on my Grandson's iPhone XR IOS 18.0.1