The MIDI THRU on the Digitone only passes through messages from the MIDI IN, not from the USB MIDI.
The MIDI OUT doesn’t pass through anything, it only sends messages from the Digitone itself.
While the Digitone can be used as a USB Audio interface, it can not be used as a USB MIDI interface.
If the synthesizer you want to connect doesn’t have USB, get yourself a simple USB MIDI interface. They also come in the form of a simple USB cable with 2 MIDI DIN plugs.
Like this one: