The Media Player of this product seems to be limited to 2-channel stereo playback according to the published Lyngdorf MP-50 whitepaper.
Even DLNA playback is limited to stereo (MP-50 fact sheet).
Media player:
Internet radio, Airplay, Spotify Connect, DLNA Support (Stereo)
Looks like you need to use another link (like for example HDMI) that supports multi-channel audio.
Update: As of the papers linked above, I’m not even sure it accepts simple PCM audio multi-channel streams over HDMI as I can’t find that listed anywhere.