i think if you are on ethernet you want to use modbus TCPIP and point to the TWIDO via the IP address you have on the TWIDO
if you want to use the primary serial port you need to use a RS-485 to RS-232 adapter and use the MODBUS RTU protocol and set the
TWIDO to have that protocol turned on for port # 1.
I know this works since I am using 6 twidos over RS485 into one HMI touchscreen on MODBUS SERIAL.