DIAGNOSIS This is almost always a 2.4GHz Wi-Fi network issue. The Core 400S does not support 5GHz bands, and certain router settings (like WPA3-only or hidden SSIDs) can block it.
FIX Ensure your phone is on the 2.4GHz band of your Wi-Fi. During setup, temporarily disable the 5GHz band on your router if possible, or move the purifier far from the router (weak 5GHz signal can confuse it). Follow the app's 'Add Device' steps precisely, holding the Wi-Fi button on the purifier until the light blinks rapidly.
IF THAT DIDN'T WORK The unit's Wi-Fi module might be stuck. Factory reset it: with the purifier powered on, press and hold the Wi-Fi button and the Light button simultaneously for about 5 seconds until you hear a beep. Then try the setup process from scratch.
HEADS UP The purifier creates its own temporary network for setup. If your phone asks to join a network like "LEVOIT_XXXX," you must allow it and stay on that network until the app prompts you to switch back to your home Wi-Fi.