@NStarche 

You are not able to designate which device connects to which band. The Deco system will place the device on a given band depending on the speeds that it needs.

What you can possibly do is to enable the guest network and enable one wireless band. On the main network, you can disable the other band. 

@Tony 

Other workaround option is, when you go to wi-fi settings, you can choose which network to make it enable whether 2.4 or 5ghz. so while setting up tv or anyother wifi devices, temporary you can disable 5ghz