In case anyone else comes across this problem; I got this email from Modix Support:
With the microSD card slot on the LCD screen, you're unable to use it because we provide the LCD panel with the 4 pin connector instead of the ribbon cable. The ribbon cable can cause issues with the usage from the PT100 upgrade that we offer with the printer, so that's why we we generally advice to use the 4 pin cable.
You can check if you have it, in more recent batches we have both cables included in the kit (I can't quite find when you purchased your printer)
You can read about the connection options here: https://duet3d.dozuki.com/Wiki/Connecting_an_LCD_control_panel