@Royplants 

Hi, network security key should be the wireless password of the router, the default wireless password is printed on the label of the router, which shows "Wireless Password/PIN".

However, if you've changed the wireless password before, you need to use the new one.

If you forget it, you could follow this guide to login the web interface of the router via another device which has already connected to the router, then go to Basic->Wireless page to check the password.