@RichHeal 

May I know do you wish to extend the network through Wireless or via Ethernet cable? Do you wish to connect tp-link extender to the router directly? 

1. If you wish to extend the network to a place which is pretty far from router, connecting router & RE via Ethernet cable would be more ideal for a stable network, at that time RE works in AP mode and wired to the router/AP.

2.  Via Wireless, when setup extender, put RE and router in the same room or very close to each other, reset extender and then set it up. 

Then relocate extender to the halfway between router or Wi-Fi dead zone. 

Here are some instructions that should help:TP-Link Range Extender Setup (Range Extender Mode)

How to confirm whether my range extender is successfully configured and in the best location

If still the same, can you share a photo of error message & the Leds on RE here?