security code for N600 wireless dual BAND ROUTER WNDR3400

There's a support site with access to manuals, firmware, install guides, etc.
Go to https://www.netgear.com/support/
Type in your model number and from there it'll take you to the above.
Its a great resource to start with.

Not much to go on there.

Why do you want to know the security code?

Tell us what you are doing and what happens and you might get better answers.