Always a hassle. Latest time I needed to copy the apk to my phone, we used to do this with adb, which is nice to get a bit acquainted with as it-student. Do this:
If adb.exe is not found you can either add the path to your apk to PATH or go to the directory containing the adb.exe and run the command from there.
Also handy:
An alternative for you might be upgrade to Mendix 9.2.0 and use the PWA (Progressive Web App). Which is way less hassle, has no vendor specific troubles and is usable on your phone without installation troubles.
Also: adding stuff to the marketplace (used to be called appstore) is free.