Hello @Bennyboy. 

Network security key is the Wi-Fi password, range extender uses the same Wi-Fi password after it successfully connects to the router.

If router's Wi-Fi password does not work, or you have forgotten the password, try to factory reset the range extender and set it up as instruction here. 

[Configuration]GuideTP-Link Range Extender Setup (Range Extender Mode)

[Troubleshooting] Unable to Connect to WiFi Extender or Saying Incorrect Password