Do you have android 8.1.0? This feature is up to android version. If you don't have restricted custom ROM by seller it must work by setting in developer options. I'm sorry it's not working on your devices. I buyed the player from AliExpress and before ordered I asked seller if it supports USB...

I think is only one version of this player. And it's about android version 8.1 https://streamable.com/mu8ek4

This is my setup.

I'm not at home right now, but tomorrow I'll put a picture. It's JCALLY JM20 CS43131

On my Z80 USB audio works, but as I said sometimes I must restart it and reselect Audio Source.

Select Audio Source and restart the player. After restarts select Audio Source again if it's not selected. Try to connect USB dac if not working disconnect dac and select MTP and then Audio Source and connect USB dac. Sometimes is not working, but this is how I manage to make it work. it...

The bluetooth connection it's stable, but I haven't tested for a long time because I don't have very good bluetooth headphones.

Maybe it needs burn-in. I listened to around 3-4 hours of music on this player. Or maybe my player is faulty or my ears :)

Yes, it supports usb audio. Only this way you can get a quality sound, using an usb dac

I recently bought this player from aliexpress and I can say that the android interface is quite fluid, you can install streaming applications, but the sound quality is quite poor without details. I am not satisfied with the sound quality.