AFAIK you have to enable "Developer Mode" on the phone and then USB
debugging.

After that when you plug the USB cable in between your computer and the
phone it will pop up a notice on the phone that warns of USB Debugging
enabled. Below that you will have a note that says it is in USB
Charging mode. Scroll down and pick File Transfer mode.
At that point your computer should realize it is hooked up to the phone
and give you some kind of prompt or automount the phone depending on how
it is set up.

I'm using Android Pie (Lineage 16) on a ZTE Axon 7 and that is how it
works for me.

HTH

Clarke