Email the APK as an attachment to the device.
On the device, click on the attachment and it will install. It will ask you to "allow unknown sources".
I've used this many times to install on devices running earlier versions of android (six, seven, or eight). I hope it still works with later versions. Sometimes you run into problems where the email server says the attachment is too big, or the type of attachment is unsafe – you might have to play around a little.
Barry.