@2385 Hello, 

1>Please verify your Wi-Fi password by forgetting your Wi-Fi network on cell phone first :  
Go to Settings-> WiFi on your phone, Tap (or Long press  the router's WiFi name  ->Forget / Delete the network 
Please reconnect to your router wifi and it will ask for a password, will it accept your router password?

2> factory reset tp-link range extender : press the reset hole for 10 secs with RE powered on
On cell phone WIFI settings, connect to Range extender WiFi first ( do not connect to router wifi or and turn off mobile data)

3> follow instructions on Tether app or Web page step by step  ( carefully examine the Wi-Fi password you have put in, and it is recommended to create a different SSID/WiFi name for RE )
 
Here are some set up videos that may help:
https://www.tp-link.com/en/support/download/re450/v3/