Hello@Deano22 

If the Leds are light up but no Internet, it is suggested to set a SSID for range extender which is different from router's WiFi first

( change Wireless settings via Tether APP)https://www.tp-link.com/support/faq/2541/

Then please follow the troubleshooting guide here to set up DHCP settings manually for the range extender as Case 3:

 RE has connected to the main router: Signal LED/2.4GHz/5GHz LEDs are on but still no internet access.  

https://community.tp-link.com/en/home/forum/topic/198516

If the concern persists despite the above step, may what is the model of your Router, please also share a photo of the DHCP settings on RE, thank you.