According to the manual, the digital outputs are addresses C17 to C24. The manual's section on modbus commands suggests that Advantech is using one-based addresses in the protocol, which would require you to use zero-based addressing in the driver. I'm not sure if that is really true.
Advantech definitely doesn't care about Modbus conventions--that model's digital inputs appear to be mapped to C1 to C8 instead of DI1 to DI8. Very strange.