@Primo Hello, 

Default login username is admin, login password is admin or it will let you to create login info in factory default mode. 

If it does not accept " admin" in lower case when you try to login, please try factory reset extender. Reset: with extender powered on, use a pin to push reset hole for 8-10 secs and most Led normally flash once.  Later refer to set up video here to login : 

https://www.tp-link.com/support/download/re650/#video

If still the same problem can you share us a photo of your login page, many thanks~