A Roku device is an external source unit that is used to stream audio/video from different sources such as Netflix, Hulu, Amazon, etc. It sounds like you are using the built-in app inside your TV to stream from Netflix, etc.
You would buy the Roku from a distributor (such as Amazon or direct from Roku website). The Roku has an ethernet port you use to connect to your internet mode. It also has an HDMI output that you would connect to your Lyngdorf. You would use the Roku as a source device just like you would use a bluray player.
I think Roku has much better streaming apps that would be available in your TV because their business is really only developing streaming hardware and software. The Netflix app would continually be updated. There are a ton more streaming apps available on the Roku.
In reality, there are several different streaming devices you could get: Roku, AppleTV, Amazon Fire, Google Chromecast, Tivo. I generally recommend the Roky Ultra because that is the only one that you can realistically upgrade with an external 12V linear power supply (such as SBooster Teradak, Teddy Pardo, Keces, Farad, etc.). Although, you would have to request a special size DC plug. The Roku Ultra uses a 4mm x 1.7mm DC plug (which is smaller than standard but still large enough to handle).