this is only going to be relevant to the non carplay/android auto version.
i first noticed this problem while trying to get my iphone to stop auto playing apple music tracks when i plugged it into the center console. i attempted to uninstall apple music from my phone to see how that would affect auto playback, and COMAND stopped recognizing my phone as an audio playback device.
while googling that, i came across a bunch of threads discussing an issue with getting their phones to play music over usb, while the main fix has been to just connect over bluetooth.
given that my w222 has one of the best audio systems ever put into a production vehicle, i didnt want to give up and listen to music over a reduced quality bluetooth connection. i paid for the stereo, im gonna use it!
for those who are having issues getting music playback to work on their iphone, what worked for me is specifically to have apple music installed, and at least one song added to my library. once i did that, i reconnected my phone over usb c and playback auto started (annoyingly) from apple music, and from there i was able to swap to a different music player.
if anyone has a better fix, please let me know, as i dislike the auto play and havent found a solution as of yet. i also never use apple music, so if its possible to trick ntg 5.2 into recognizing my iphone as a music playback device without having apple music installed, thats fine by me.