Heltec HTCC-AB02 modbus RS485 RX problem

Hi. I am using a HTCC-AB02 board to interface with a modbus RS485 sensor. By using modbus poll software, I can receive command from the HTCC-AB02 and send a command back to the board. However, the board is not able to receive the command from the board. May I know what is the cause? I am using Serial1 (TX1 and RX1 pin)