@TotoWax I tested in Plex and indeed, Plex dashboard shows the true bandwidth is almost double. I think I'm in the same boat as you, I just tested a movie in 4K HDR on MP4 container (same as you) with 'only' 100mbps bandwidth and it doesn't play smoothly. 4k HDR on MKV or m2ts containers is no problem so I think it's not a network thing, more a Plex/player/compatibility/transcoding issue.
To answer your question about the router: I had to turn of quick roaming (IEEE 802.11r) otherwise my TV couldn't connect to the network. Also turned off Beamforming. Those are the only things I could change as it's setup as an Access Point. I have TP-Link Deco M5 units.