Maybe send a screenshot or video because if files & browser have permission to install the only other thing I can think of is checking to see if googles play protect is stopping you from installing because it should work. It does on my moto device the g stylus 5g
Discussion
I meant of the permission screen. It is also possible that the apk is not a universal coded app for it to work on any device maybe it only supports armv7 or armv6. If the popup is coming up would you like to install & you can hit install then yes you did the correct steps but if it won't install after hitting install then create an issue on the github & give them thenfull specs of your device.
Just showing it works on this Pixel 5a
https://video.nostr.build/9ec2a7489567068d297248d24ac20043ba5c283fdb9414f1618137d9777e6a85.mp4