Your board would have come with an SD card that has a dual partition setup. One partition will work when it's inserted into the Duet, and the other is a partition for the Pi when inserted there.
If you still have to original SD card and it hasn't been modified, insert that SD card into the Duet and disconnect the Pi ribbon cable.
That should allow the Duet to be reachable if you follow the getting connected guide.
https://docs.duet3d.com/en/How_to_guides/Getting_connected/Getting_connected_to_your_Duet
If you SD card has been modified you will need to prepare a fresh SD card.
https://docs.duet3d.com/en/User_manual/RepRapFirmware/SD_card