How to use Dual channels

Hope someone can help me understand, the documentation is scant on this. I have a WNDR3400v2, it has channels for 2.4Ghz and 5.0Ghz. Both lights are on, but how does one connect to the 5.0Ghz channel? Is it automatic? I almost thought I would see to access points on my router list to chose from.

I am sure this is pretty basic stuff, it is all just new to me.

Thanks
Steve