Well, I have finally managed to increase the network throughput to at least 150 Mbps, which is fair enough for my initial problem - 4k video content with bitrates grater than 100 Mbps. My good old wireless router (DIR-600) had behaved very weird in the last weeks - the internet speed on my computer was very slow, only half the bandwidth I receive from my ISP. So I decided to buy a new dual band router (Archer-C5 V4) and now the computer receives full internet speed, and surprisingly the TV (only wifi N) connects to the 5 Ghz network at speeds > 100 Mbps.
Now the move in case (American Made) plays very smooth without any stuttering - max bitrate measured with speedometer on the Plex linux box was 15 Mb/s, or ~ 120 Mbps. Another movie with peaks > 100 Mbps is Life of Pi, especially the scene with thousands of moving meerkats on the island.