Hello  @Mbbeaudet 

Sometimes if you have pressed the WPS button to set up the range extender, RE will copy the SSID from main router  and looks like range extender Wi-Fi SSID disappears. 

If the Wi-Fi LEDs/ Signal leds are OFF and Range Extender default Wi-Fi SSID does not show up and   try to factory reset the RE first. 

• RESET: With the extender powered on, use a pin to press the RESET button (pinhole) until all the LEDs flash once.

Double check on a different Wireless Network device like phone or laptop and see if there is any Tp-Link range extender XXX or Extender XX Wi-Fi SSIDs? 

You could get more details about how to reset and configure the range extender to the guide here:  

TP-Link Range Extender Setup (Range Extender Mode)