Setup is a QNAP TS-251A
1Gb internet connection
fully wired (cat 5e) setup to a new Sony KD-55AF8 Television

I've been trying to stream 4k content from the QNAP to the TV.

I've tried
Kodi (on the TV)
Plex (on the QNAP)
VLC (on the TV)
I've even tried the Sony media player via DLNA to the QNAP
and various other so-called media players.

For the record, Kodi is my system of choice and plays my 1080p content flawlessly.
I have disappointingly found out the TV does not have gigabit network hardware built in, but I've also tried wireless which I was suggested would get me better speeds/playback.

However on high bitrate 4k files (80Mbps) no matter what I do, I'm getting varying levels of buffering and stuttering playback (depending on what media player I'm using).
4k playback via the internet (youtube etc) plays fine, so why can't I stream 4k from my NAS properly?

Any clues or things to try very much appreciated.

Is it possible to plug a gigabit network adaptor into the TV via USB to give me more bandwidth?