I have this problem before. I tried to install USB drivers for my phone as when I hold down the C button while connecting it to cable to pC,it looks for usb drivers.
but it didnt detected in phone xs. so i restarted by phone and PC. launched the phone xs+ click the start button. upon seeing the "30 seconds" mesage I attached my phone while holding C button. then it was detected. try it. it might help..