This article describes why the Modbus communication does not work over RS485.
Why isn't my Modbus communication working over RS485?
Often there is a problem with the wires. First thing to do is to check that the A and B lines are connected properly. Next thing is to switch the A and B line and see if it works. For the Netbiter A is + and B is -. Other things to troubleshoot are the Modbus communication settings on Argos, baud rate, parity, stop bits, slave id etc.