You can't change the volume from set top box once you enable "Dolby Digital". There is no way to do. You can't do it. It is a digital encoded signal which does not allow to change the volume. You can't do it on any box, whether Tata sky or Android TV or anything. Dolby digital signal is "passthrough".
Since you are not using a surround sound receiver, there is no use of Dolby digital for you. Just change your settings to "PCM" and then you can control the volume through set top box.
No software update can change this. It is neither possible now, nor will it be possible in the future. If you enable Dolby, you can't control the sound from the settop box. Dolby passes the original audio to your TV or receiver, which controls the volume.