At the moment I can only connect to the Modduox via my laptop.
With the desktop the connection fails, no matter which USB port the Modduox is connected to - I have tried them all. I used Firefox and Chromium, I had the extensions disabled.
What does lsusb on the desktop say when the Duo is physically connected? If the Duo is detected you will see something like 0525:a4a2 Netchip Technology, Inc. Linux-USB Ethernet/RNDIS Gadget. If this is not the case, we got a problem on the USB hardware level.
What network devices are present when running ip a? Usually the last device in the list is the Duo. Does it have an IP address assigned? If there’s an interface with no IP, is the network manager running?
Also try running sudo journalctl -fk before plugging the Duo into the USB port and look at the USB and network related messages.