Hello @mollydarby 

If the Sigal Leds are still OFF after set up RE, that means range extender has not connected to router wifi network yet to, it's a great step to verify router wifi password first as guide here , then factory reset RE and set it up from scratch TP-Link Range Extender Setup (Range Extender Mode)

Here is a troubleshooting guide that might help:Has connected to TP-Link extender but no internet, what can I do?

If still the same, may I know the model of router, the Status of range extender web page after set up, did the verify the wifi password of router these helps us to look into the case further.