Looking at the NG9 Modbus registers, I see that ID in the table is the value of Modbus address. However, in RUT955 you have to enter Modbus register number, which value is one higher (+1) than address, try to use 7007 for Voltage and 7501 for Active Energy. After this you have to get value after pressing "Test".
Just to make sure, after changes first save configuration and then get back to press "Test" button. Since only after saving values are loaded. Also, don't forget to turn off RS485 device in Services -> RS232/RS485 -> RS485, since you have mentioned, that you tested Modbus gateway.