I gotta say, I'm having the hardest time getting my iMac to see the N5ii's internal storage as a USB drive. Nothing I do gets my computer to recognize it, even when the prompt to turn "USB storage" on appears. I can't think of any other way to get the firmware update onto the device itself...
Which OS version do you have?
I have the lasted one and I can mount the microsd card like an external drive, very useful and much easier than using a specific app.
I would also recommend to try with a different usb-c cables, I had one in teh past that did allow me to connect the player to the Mac.