If you get CAT control works with another interface and does not work with the same settings using Digirig’s serial port then there must be some hardware or compatibility issue.
You are welcome to send the gear back for re-testing and if everything checks out then I can issue a refund instead of sending back the replacement. Please PM me for arrangements.
BTW: Do you configure the serial port for 1 or 2 stop bits? I would try an alternative setting there.