SBC mode requires removing the SD card from the Duet board, so if the SD card is physically there, but it still detects as being in SBC mode that indicates something may be wrong with the SD card slot, or the SD card itself.
When the Duet is in SBC mode the Pi does the networking, so network commands aren't accepted.
Sorry about the dud SD card. You might be able to do a full format of the card and get it working again.
Now that you're up and running again you can follow through the getting connected guide: https://docs.duet3d.com/en/How_to_guides/Getting_connected/Getting_connected_to_your_Duet