Hi  @Chienito,

If your TP-Link range extender was working fine before, but it failed to connect to the host router or lost Internet connection suddenly, the following troubleshooting steps can be helpful: 
    1. Try restarting both the router and the extender. 
    2. Ensure the router's Wi-Fi network is working properly. 
    3. Ensure that your extender is within range of the router.
    4. Keep the extender away from interfering devices like metal objects, fish tanks, microwaves, Bluetooth devices, and so on.
    5. Use a pin to press the reset hole for 1–8 seconds to restore the extender to factory default. All lights will turn on momentarily when the extender resets. Then launch the Tether App, click the top right "+" button to add it. Check this link to view comprehensive instructions: 
How to configure Range Extender via Tether APP

If the issue persists, please contact your local support for warranty consultation.