Topic: 5.1 not working with Logitech Z906 speaker system and optical cable

I have a Logitech Z906 5.1 speaker system and a PlayStation 4 Pro, and I am only getting stereo sound.

Here is a link to the Logitech Z906 manual:
https://www.logitech.com/assets/36226/z906620-002920006ug403.pdf

PS4 Pro settings

Settings > Sound and Screen > Audio Output Settings >
Primary Output Port > Digital Out (Optical) > 'Dolby Digital 5.1ch' and 'DTS 5.1ch' selected
Audio Format (Priority) > 'Bitstream (Dolby)' (selecting 'Bitstream (DTS)' did not help)

Speaker system settings

There is no HDMI port as it is not an AV received, so I have the optical cable connected to 'Digital optical' port 4, and I have my speakers set to Input 4.

Game settings

I only have one game: Uncharted: The Nathan Drake Collection, and all settings in Sound within the game are set to 5.1

The forum will not let me edit posts.

I changed the optical cable and now it has gone from stereo to 5.1.