Hello@laussana
May I know how about the Wifi lights status on RE505X after set up ? If that are OFF, range extender has not connected to router WiFi yet, please try some suggestions in thread below and let us know if it works for you:
Has connected to TP-Link extender but no internet, what can I do?
Please verify your Wi-Fi password by forgetting your Wi-Fi network on cell phone first, later reconfigure tp-link range extender.
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?