@Markwilliams Good day,

You could try some suggestion here: 
Case1:  RE Led/ WIFi Leds are OFF on the range extender, and wireless client will not accept the password of router

Mostly likely that users have put in wrong password during the RE set up, please factory reset RE and reinstall it from the beginning, put in correct wifi password during set up
Reset:with RE powered on, use a pin to push the reset hole for 10 secs until most Leds flash once

Case 2: RE Led/ WiFi Leds are ON on the range extender, and wireless client will not accept the password of router

Download tp-liink Tether app, check Range extender's password on Tether app:  
https://www.tp-link.com/support/faq/2541/

If wireless device reports an Error when connect to Wi-Fi, delete Wi-Fi profile/ Forget Wi-Fi on that device and put in Wi-Fi password again. 

If still the same problem, may I know can other wifi device connect to RE using the same password?

What is the model of your smart YV, and may I have a photo of the Error message ?