Can’t output 7.1/5.1 audio from an iPad Pro 2018

hi there
i use the ipad pro 2018 to play the movie,
when i use the hdmi pass through that can’t output the lpcm 7.1 audio.
because,the hdmi 7.1 audio decoder can’t receive the lpcm 7.1 singal,
the analog 7.1 output port it’s only 2 channel(FL,FR port) can hear the sound.

then when i use picture in picture to play the movie,the hdmi 7.1 audio decoder can received the lpcm 7.1 singal,every channel’s sound is fine。
so my point is
i need to play the movie in regular mode,and output the lpcm 7.1 sound to the hdmi audio decoder ,and then i can connect to my 7.1
home theater speaker,that’s will be perfect!
so can add something to make this work?

hdmi 7.1 audio decoder like this