Hello @Confusednumber1 

Can I know the model of the wifi router, its on router sticker. 

Range extender should use the same password & wireless security as Main router after it has been set it up successfully, if you found RE and router does not use the Wi-Fi password: 

Case1:  RE Led/ Wi-Fi LEDs are OFF on the range extender, and wireless client will not accept the password of router

It is possible that  wrong password has been put in during the RE set up, please factory reset RE and reinstall it from the beginning, put in correct Wi-Fi password during set up
Reset:with RE powered on, use a pin to push the reset hole until all the LEDs flash once

Case 2: RE Led/ Wi-Fi LEDs are ON on the range extender, and wireless client will not accept the password of router

Download tp-link Tether app, double check Range extender's password on tether app and use that password to connect WiFi :
https://www.tp-link.com/support/faq/2541/

If your mobile device could not connect to RE's Wi-Fi, please try to log in on your computer using ' tplinkrepeater.net'  or IP address of RE. 
Try to find RE's IP address on router's Client list.