Downlink message from server to node

I have a RAK7431 on hand and connected successfully to TheThingsNetwork, the uplink messages are okay, however, I meet a difficulty in donwlink message.

The downlink message was successfully sent from server to RAK7431 but we didn’t get any output from the RAK7431 modbus output.

Are there any settings to achieve the downlink message to modbus output?

View the log from the device using a USB cable. What does it show?