Hello @Yogace 

If you use WPS button to set up extender, it will use the same SSID as the main router.

May I know how about the Led status on range extender now?

1. If Wi-Fi / Signal LEDs all light up on the extender:You could change wireless settings as guide here

How to change wireless settings of TP-Link range extender on Tether APP

2. If Wi-Fi / Signal LEDs are OFF on the extender:

You could factory reset extender and install it from scratch, after factory reset the Extender SSID will show up. 
RESET: With the extender powered on, use a pin to press the RESET button until most LEDs flash once.