Why is there no reponse from ModbusRTU slave using converter?
Try register 515 instead of 40515.  the 4x prefix is just to say "holding registers"  If that doesn't work, try 514, for register addresses vs register numbers.

The "comm light" can blink just because it received a modbus request it understood, but had no answer for.  It's nice if devices reply with "illegal register" and it's in the protocol docs, but you can't force devices to follow it.