I think that the Subnet and Router settings in the IO Devices are syncronized to the IO Controller, even if it does not appear so in the dialog.
In TIA V15.1 I have a setting for each device "syncronize router settings with IO Controller". It is set by default, and the subnet and router settings of the IO Device mimic the IO Controller. I am guessing this behaviour is the same as V14, you just dont have the option to clear the "syncronize router settings with IO Controller" setting.