I've always been able to use my BT remote to control the overall volume of my surround sound with my old BT youview boxes.
With the pro this doesn't seem possible??
HDMI cable box to TV.
Optical cable surround system to TV and/or box.
The remote just turns up the TV volume not the box.
I think the problem isn't with the BT Pro Box but with your sound system connection to your TV.
I have a similar setup and it works for me. The BT remote operates the TV sound, if the TV is connected correctly to an external sound system then I think it's the TV that is then controlling your sound system volume.
On rare occasions my TV seems to forget the optical connection to my soundbar and I'll experience the same problem as you. A quick delve into the TV Sound Settings to make sure Optical Out is selected for the sound and it's sorted.
The remote works the TV volume but my old BT boxes had their own sound level so I kept my amp at a certain volume and used the BT remote to control the sound i.e. the box sound not the TV.
Press the Home button on your BT Remote, then select Settings (the cog in the top RH corner).
Then scroll down to Bluetooth Remote and select. Then change 'Control Volume and Source' to Set Top Box and choose Done. You should now be able to operate your system as before.