Your case (if I read and understood correctly):
1 - everything works without
https (self-signed-certificate) on Android, PC and iOS
2 - with
https (self-signed-certificate) all (navigating and downloading) works on PC and iOS, but downloading does work in Android
To me (and google) it seems a problem of Android with self-signed-certificates.
You could try to install
Let's Encrypt on your server. It solves the problem of a self-signed-certificate.
I have neither Nas4Free nor Android (an Android VirtualBox refused to work on my outdated hardware) so I cannot help you testing.