Personally, I think that the Code you'll need to put in is not inserted correctly. Some notes to be aware of: WPA is Case Sensitive, so Apple is not the same as APPLE, WEP encoding only uses numbers and the letters A to F. Depending on the router, the default code is often on a sticker on the router. However, this code can be changed by someone with admin access to the router, some of the details sometimes for admin login are also on the same sticker, assuming it isn't changed as well.
Another option, which that usually a Parent/admin will setup is to only allow certain computers online, by MAC filtering. You would need Admin access to the router and the MAC address from the 3DS settings app on the 3ds you want to connect.
Getting admin access to the router is not standardised and I recommed looking at your ISP help site, or router Maker's help site.