I recently found this "broken" Roland GAIA SH-01 on a local Facebook Groups.
The seller reported the Synth as "broken", with no sound coming out of it and only some noise on the outputs. I tested the unit and everything else was apparently working, all buttons worked, all leds were ok and so on...I suspected a problem with the DAC or with the amplification stage of the synth, but took it home anyway presaging a very quick fix.

Once at home the first thing I suspected was that the keyboard was simply deactivated (local mode off). I did a quick Factory Reset of the synth, but still no sound. (To reset, turn on the unit with both PRESET PATCH and USER PATCH pressed, and then WRITE. To simply switch the "local" hold down the CANCEL/SHIFT button and then press the CENTER CANCEL button. When the button is lit then local is ON).

The I tested the AUDIO IN and the sound was going out of the synth and volume was working. So, I excluded an issue with the amplification stage.

I then connected an external MIDI keyboard and, with greta surprise, the synth was working. So, I excluded a problem with the DAC.

The last option I considered was a broken keyboard, or a broken keyboard connector.

I opened the synth and with great surprise (and pleasure) I found that the keyboard flat cable was simply disconnected.
I still don't understand how on earth a 40-pin flat cable with an IDC connector could have come off, but anyway...I "fixed" it!