@OrTaddei 

Hi, you could start with resetting the extender first:

How to reset the range extender to its factory default settings:

With the extender powered on, use a pin to press the Reset button (hole) for 10 seconds. The extender will reboot.

After that, you can refer to this link to set it up again:

TP-Link Range Extender Setup (Range Extender Mode)

Thank you very much and best regards.