Get yourself a Raspberry Pi, version 3 would do but they have just released the 4 with up to 4GB of RAM.  Then install DietPi which makes the set up Plex Media Server and other programmes really easy, add an external drive and you should be set to go.  The format of the media should not be a big issue as the Plex Media Server will encode it in a format suitable for the device seeking to view it. I have watched 480p 720p videos of not particularly great quality streamed to my 2016 49" LG from a RPi 3 and the results have been more than adequate.  100Mbps networking should only be an issue with 4K if the media have been encoded at a really high bitrate/quality etc.  If the cable is not quick enough then maybe it is time for faster WiFi.

I solved from myself using the suggestion of another forum. Hotbird 6/7a/8 is now called hotbird 13, so the presetted setting was good. My connector was broken, i have redone it and now the preselection works great,

My internet up/downloadspeed is 100Mbit but all the computer and the LG TV are connected by wired network (100 Mbit or 1Gbit (don't really know)). Normaly I stream 720p movies (because of the discspace) but it works without problems with a 1280p movie file. The filetypes you can stream can be found on the Plex website.

how fast is your internet speed? Do you connect through wifi or cable? What kind of movie files do you stream? 

Hey LG mates, I have just bought OLED C8 and I am reading all the articles about streaming movies from PC to my smart TV, but I am not successfull at all.  1. I have 1Gbps internet, but all of my devices are connected to Wifi. Speed tests display 100 Mbps for download and upload maximum so I suppose that the router is not the best one. I have router right behind the TV, so maybe this will not be the best place after all. Should I use LAN cable for both devices or only one of them? I have bought CAT6, but as I read that the TV supports only 100Mbps through LAN cable, I am not sure if this will help for 4K movies.  2. I downloaded Plex to my MacBook Pro 2017 and my LG as well, but I was not able to stream anything. 1080p stopped in the beginning and was buffering for more than 15 minutes so I closed the application. What should be the proper set up of PLEX? 3. What preferences should the movie file have?

@KM Lee Did you run some internet speed test to see the difference?

Guys, is it possible to use subtitles within your solution?

I will look again.