Did you ever find a solution to this? I am having exactly the same problem with the 6121MFP.
The computer recognises the scanner/fax but doesn't recognise it as a printer.
Just inherited a Phaser 6121 from a colleague who has upgraded. She has used it with no problems for a number of years.
I'm running Windows 10 x64. I've tried for hours to set it up as a printer on my laptop to no avail. I have got the scanner working.
I'm using a USB cable as I just want it as a local printer to my laptop. Have also tried with ethernet but no joy.
Have tried downloading drivers but nothing happening.