Heyo. I did the same thing last week. If you want to avoid wiring, it is possible to control the robot with only the ethernet cable. You will have to create your own communictaion protocol, and not use the default Modbus one. The robot will only accept the protocol 0x00 0x02, while the default Modbus one is 0x00 0x00.
At least, this is what i discovered when i tried to use a Siemens PLC.