@Pappymom,

If you are having to set up a new router and modem, it is recommended that you factory reset the range extender and start the configuration process over again. This way you can be sure that the Range Extender has the most up-to-date settings and is connected in the fastest way possible.

A User Has Previously Posted These Instructions on How to Reset a Range Extender:

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If you are facing a problem with resetting a TP-link extender you can try this process press and hold the reset button for 5 sec or until the WiFi LED blinks amber and green to initiate the app configuration process. To start switch/plug to factory defaults: Press and hold the reset button until the WiFi LED blinks amber rapidly (about 10 sec ) to reset the smart switch to factory defaults.                        

if you are still facing a problem you can follow this process:

1. Make sure the range extender is plugged into a power outlet.
2. The power light on the front should be lit solid.
3. If the power light is not on, press the power button on the side to turn the device on or the LED button to turn the indicator lights on.
4. Locate the pinhole on the side where the reset button is.
5. Insert a pin then press and hold the reset button until all indicator lights turn on momentarily.
6. The range extender should now be reset to factory default settings.

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Otherwise, we will need more information regarding what products you are using. Hope you can get it working soon!