Is my connection to slow or another issue?

I have a 40meg Sky connection and get perfect sync both upload and download.
This is a speed test i did not long ago.
speed test

I have done repeated tests at all different times in the day and they remain around that area so i am guessing my connection is pretty stable. That drop near the end of the test was Chromecast loading a new picture for example. I saw it change pic right as the speed test was ending.

So here is problem.
Both my 4K TV's app and a 4k Chromecast ulra are failing to do 4K YouTube. They can do 4K Netflix and Amazon and my TV even did the TV 4k Iplayer demo yet almost all YouTube videos fail.
I should point out that Youtube 4K works perfect on my PC's

So what is going on here. Is my TV and Chromecast Ultra too slow to cope with what they designed for with Youtube? Or is my connection too slow because of the less amount of ram to buffer they will obviously have compared to my PC's

Another possible cause in my mind could be this TP link switch, There is something about it i don't trust it's lights can act lets just say "odd" sometimes.

I would love thoughts on this. As specially now a 2nd device a Chromecast Ultra is failing just like my TV's app. Remember this is only Youtube. Even Amazon 4K HDR is working with ease. Loads really fast.

I'm pretty sure it's not you as my TV is also having problems. A LG 2016 model. It refuses to load/buffer.

I can watch this on my mac https://www.youtube.com/watch?v=6pxRHBw-k8M and this https://www.youtube.com/watch?v=bZBOx5lPAu8 but not on the TV

My TV has about 30 4K naughty channels and they are all working fine - so it's more than likely a problem with YouTube

I don't know. Reviews of the Chromecast Ultra said that on a 80meg connection it was playing 4k HDR Youtube videos perfectly.

All depends on what bit rate the YouTube stream is trying to use, and that is the unknown it seems

I got one of them videos the person earlier posted to play at 4k on my Chromecast ultra all the way through (very rare for me). However it went to 4K then back to HD on TV app itself.