Starting from recent firmware versions (>7.13), there was a change in the usage of the ubus modbus command, and it’s now recommended to use the API calls for reading Modbus register data.
However, there’s a known workaround method involving two ubus calls that you can use to achieve similar functionality. This workaround has been outlined in the topics here:
Update:
It seems that on the latest firmware this ubus call command works properly, as like on previous releases (<7.13); hence, if there’s a need for its usage, please update your device’s RutOS firmware to 7.14.