maybe the HDMI2analog I bought is just crap.
Maybe this one is more suitable? https://www.amazon.de/ROLINE-Extraktor-Stereo-Converter-Unterstützt/dp/B00VFNYBM4/ref=sr_1_1
If the Hdmi2Analog connected to the Oppo works only as stereo, it means it does only ac3/dts decoding. As i said, the description in the website you linked wasn't so clear.
Also, in the description quoted above (just seen it now) it says up to 96K, that's the ceiling for DTS, hdmi should go up to 192 stereo. It's the chipset inside the extractor that doesn't handle the audio fully.