@Charalie  Hello, 

Most of the time range extender set up failed (leds off) due to the incorrect wifi password was put in during the installation,  can you help verify your Wi-Fi password by forgetting your Wi-Fi network on cell phone : 
Go to Settings-> WiFi on your phone, Tap (or Long press  the router's WiFi name  ->Forget / Delete the network 
Please reconnect to your router wifi and it will ask for a password, will it accept your router password?

This is also a guide to troubleshoot RE installation step by step, kindly give it a try and let us know if it works for you,
Has connected to TP-Link extender but no internet, what can I do?
https://community.tp-link.com/en/home/forum/topic/198516