YouTube app on smart TVs

To lay out the situation: I have an Apple TV, and my home internet speed is sitting at 20 Mbps—which, frankly, is quite fast and reliable enough for most tasks.
I’ve started noticing that the video quality occasionally drops significantly when I play a clip on the TV, performing much worse than it actually should be, considering the video is available in both 720p and 1080p resolutions.

When I cast a video from my phone, I noticed earlier today that you can actually tap those three little dots to manually select 1080p, which immediately fixes the quality on the TV. However, now, when I attempt to watch that exact same clip, that specific option has vanished entirely. Why on earth is that? All I see are the Subtitles and some other setting—maybe something regarding blocking? The ability to choose the resolution is simply gone.

Is there some way to force the system to always default to the highest available quality? In my case, since the TV isn't 4K, I just want it locked at 1080p. And why does this happen inconsistently? It’s the literal same video every single time, yet the options change.

"You can't really force the quality to stay at a specific level because YouTube on smart TVs is designed to be pretty smart about it; it constantly adjusts the resolution based on how fast your connection is moving at that exact moment. In a perfect world, you should see things sharpen up after about ten seconds or so once the stream stabilizes, provided nobody else in the house is hogging all the bandwidth by streaming movies or downloading huge files at the same time."

Alright, thanks for the help. Sometimes it feels like the quality just gets stuck at 720p, whereas my actual intention is to have it locked in at 1080p whenever possible.
It honestly remains a complete mystery to me why I was once presented with the option to manually select the resolution while watching on my smart TV, only for that functionality to vanish into thin air immediately afterward?

Since you only mentioned seeing that video quality option "once," we aren't exactly mind readers who can point out which specific button to press on your remote control 😁 How far away is your router from the TV? If it’s tucked away in another room, it’s really no wonder you're struggling to hit the highest playback quality—especially since 20 Mbps isn't exactly blazing fast by today's standards, considering most US providers are pushing at least 35-50 Mbps as a baseline.

It’s not even about some button on a remote control; I am casting everything directly from my smartphone. It gave me that selection menu just once before, but now? Nothing. Why on earth would it stop doing that? 🙂
So, here is the workflow: you start playing the video on your phone, you tap those three little dots in the top right corner for options, and normally there should be a choice. But lately, when I try, the menu only offers Subtitles and one other thing—there is absolutely no option for quality settings anymore.

And don't even try telling me it's a bandwidth issue; even at a mere 20Mbps, it should be plenty because my connection is rock solid, and I can stream 1080p on my laptop without a single stutter or hiccup.
The catch is that the router is actually in a different room—it's sitting on the floor in the upstairs bedroom, whereas the TV is mounted on the wall down below 🙂. Regardless, I specifically invested in a high-end Cisco router precisely to ensure we had seamless coverage throughout the entire house.