Trying to connect ZD410 via USB to iPad lightning connector

I'm trying to discover and print to a Zebra ZD410 printer from an iOS program running on an iPad, using a physical connection between the iPad lightning connector and the USB port on the printer.  My app is using the External Accessory framework, and I have specified the "com.zebra.rawport" protocol, which works for Bluetooth classic communication.  However I am unable to discover the Zebra device; that is, it doesn't show up in the list of connected EAAccessories.  Is there perhaps a different protocol string to use for this application?

Thank you.