@kalanbays,

Is your modem able to broadcast a wireless signal? If so, you would have a gateway, which would either need you to use AP mode, or enable bridge mode on your Gateway.

As long as you are connecting to just a modem and not another router - you may leave your Deco in Router Mode.

The Switch is entirely optional, it simply adds additional ethernet ports for you to work with, such as if you needed to wire a PC, server, or other devices on top of your satellite nodes. You may always consider adding a switch to your configuration at a later time if you find that you need more ethernet ports for your connections. The addition of a switch will not change anything about your network, and your nodes will be able to reconfigure themselves to communicate with the main node.