Hello Jack,
You may need to go through the steps one more time in driver properties and make sure the the duplex module is set to "installed" as stated here in the steps.
I have recently installed a Phaser 6510 printer on to a Window 10 work PC and it appears to be working as intended with the exception of the duplex printing.
I have set the Device Settings in Printer Properties to "Optional Duplex Unit: Installed" however when I print the Startup Page from the printer is says "Duplex Module Not Installed".
I have removed the printer and tried to reinstall without success.