DIAGNOSIS The Wi-Fi module in the purifier has probably lost its connection settings, or there's a conflict with your home network.
FIX First, reboot your router. Then, on the purifier, you need to put it back into setup mode. Press and hold the "Airflow" and "Oscillation" buttons on the machine's base simultaneously for about 10 seconds until the WiFi icon on the display starts flashing. Open the Dyson Link app and go through the "Add a machine" process again as if it were new.
IF THAT DIDN'T WORK Ensure your phone is connected to a 2.4 GHz Wi-Fi network (the TP07 doesn't support 5 GHz for setup). If you have a mesh network or multiple access points, try moving the purifier closer to your primary router during setup.
HEADS UP The machine stores connection data locally. If you change your Wi-Fi password or network name (SSID), you will have to go through this re-setup process.