@IanRoberts 

Hello,

When you plug in the range extender, which light is lit up?

Normally, when you plugin, only the power light on, after the configuration, all the lights should on.

So you can try to set up the range extender again.

Please put the range extender and your main router in the same room, then please click on this link to get detailed information: 

For Single-band RE:
https://www.tp-link.com/en/support/faq/1411/
For Dual-band RE:
https://www.tp-link.com/en/support/faq/1400/
                                                                                                                                                                                                                                                              NOTE: During the wireless settings process, it is NOT recommended to "copy host SSID". Please create a new SSID/ WiFI name for your Range extender. 

You could connect to the new SSID using the main router's wifi password to check Internet access.

If it still can't connect successfully, please refer to this FAQ to troubleshooting:

https://www.tp-link.com/en/support/faq/2234/

If you need any further help, please let us know which step you met trouble, the Error message, and send us a photo of LEDs on your range extender, thank you!

Please find the model number at the bottom label of the product, which may begin with TL-xx, RE.xx, or Archer xx. Please also find the hardware version of the product, which begins with Ver.xx.

Have a nice day!