It should respond to UART commands without any connections. Just the bare board.
I think yes it does. If I send text over serial monitor it appears in the console. I don't have experience with Arduino, sorry
I needed load a serial sketch to do that
ps.
i think maybe the problem is the wiring, do you know which pin at the external dmm connector is #1?, i mean seeing it frontally , the red wire at the ribbon cable usually mean pin 1 as far i understand, but.. seeing the schematic pin 1 at the socket is connected to ground, that don't make sense to me. or at least hp mount the internal connector backwards to avoid the ribbon be too near the transformer. in that case pin 1 really is at the other extreme of the socket (i mean near to the notch of the socket, usually pin 1 is at the notch side). or the socket is inverted, i am confused.
just an idea
Best regards
Pio