It doesn't appear your TV supports surround sound. Most TV sets "dumb down" multi-channel audio to stereo because TV sets only have 2 speakers.
You will contest this with the results of your Netflix app. Technically, your TV isn't decoding the audio from Netflix (Dolby Digital Plus)...the Netflix app is either decoding or simply passing the audio data to the optical out.
Instead of connecting the cable-box to your TV, I'd connect your cable-box to your A/V receiver via HDMI. That way, your A/V receiver becomes (will become) the switching center for all your A/V components.
To get video from the A/V receiver, connect another HDMI from your A/R receiver to your TV.
Because your TV doesn't have HDMI-ARC, leave the optical connection from TV to A/V receiver.