I was unable to find a plugin that would allow me to play custom streams, so after days of learning Python, with help of shopgirl284, Mikedm139 and examples found on this forum, I created my first Plex plugin; I present You Plex plugin that plays network streams (a.k.a. IPTV) from a M3U playlist:
https://github.com/Cigaras/IPTV.bundle
Also available in Unsupported AppStore V2
OneTechGenius also made a video guide on how to set up various Plex channels including this one, You can find it on Youtube, a written version is also available.
Included sample playlist is outdated. You can find some playlists scattered along this thread (like posts #388, #399, #407, #417, #457, #500, #501 and so on) or just google for them.
At the moment plugin supports HTTP and RTSP streaming protocols, RTMP also works with some limitations, and MMS did not work for me on any device, some users report that it works in Android and Plex Web.
Please keep in mind that this plugin does not actually play videos by itself, it only passes links to Plex in a format understandable to Plex and the playback is done by Plex client, and it is a known fact that Plex Media Server does not transcode live streams so you might need a third party transcoder like VODServer if client is unable to transcode streams by itself, please read post #97 (especially Apple TV owners) and compatibility section of readme file for more information.
If plugin does not work for You or You don't like it or don't like Plex, but still want to watch IPTV on your Smart TV or other device, You can check post #194 for alternative solutions, but in any way please consider making a small donation if I was of any help for You.
And instead of summary, if someone of more experienced developers could spare a few minutes to test and review my plugin I would be more than grateful, please help me perfect it to be suitable for the App Store.