Diagnosis
The Wi-Fi module in the purifier has lost its connection to your router. This is common after router reboots or Wi-Fi password changes.
How to Fix It
Factory reset the purifier's Wi-Fi. On the machine itself, press and hold the power button and oscillation button together for 10 seconds until the Wi-Fi light flashes. Then set it up as a new device in the Dyson Link app.
If That Didn't Work
The purifier only works on 2.4GHz Wi-Fi networks. Ensure your phone is connected to 2.4GHz during setup. If your router combines bands (dual-band SSID), temporarily disable the 5GHz band to set up.
Heads Up
The Wi-Fi module can overheat. Ensure the purifier is not placed in direct sunlight or right next to a heat source. Poor signal strength can also cause intermittent drops.