If all you need ethernet for is your TV and the Roku device, then the ethernet splitter may be all you need. These come in both a powered option and an unpowered option. With an unpowered splitter, one less cord to plug in. I generally use an ethernet splitter when needing to transfer files between a couple of laptops/devices.
However, I prefer an unmanaged switch for a network due to its expandibility and a few features it has that is lacking in a splitter, but for the most part, a splitter and switch are pretty much the same. Switches though require power so there is that cord there.
But using the splitter, or switch, you can connect both your Streambar and your TV. (independent communication). Both provide 1 Input, multiple Outputs. Of note is that both splitters and routers come in different speed specs. The Streambar will still be limited to Fast Ethernet (100mbps), so unless your TV specs out faster than this, you can consider whether you need 100mbps or 1000mbps when shopping. (these should be backwards compatible, but there is always a chance of compatibility/operability issues).
From the splitter or the switch, you would run one ethernet cable to the TV, and the other to the ethernet adapter which would then connect to your Streambar.
For your ethernet adapter, I would propose you consider an OTG ethernet adapter. For the same price as a standard ethernet adapter, gives you the additional capacity to add a storage device (USB flash drives, thumb drives, etc) that can provide the ability to access your local content on your Streambar and Roku devices through the Roku Media Player. (SMAYS OTG - compatible w/ Roku )
As for your original question though about ethernet and disconnecting wifi. While the Streambar would be connected to your network via ethernet adapter, the Roku remote would still be communicating with the Streambar via a private wifi network. You would need to switch over to an IR/Simple Roku Remote (Players Remote) in order to be able to disable this part of wifi broadcast. https://www.roku.com/products/accessories
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Essentially your Setup would be Router -> Splitter or Switch -> Ethernet Adapter -> Roku Streambar.
