Consider Infuse instead of Plex. It’s a better app for the purpose in my opinion.

Are all devices updated to 17.1?

Alternatively, check this list of 4K movies on the Apple TV app. The list is updated regularly: https://www.flatpanelshd.com/news.php?subaction=showfull&id=1505495519

Apple TV HD?

You didn’t even try to Google it?

As for NordVPN, they have confirmed that they are actively working on an app for Apple TV.

Here’s a full list that is getting regularly updated: https://www.flatpanelshd.com/news.php?subaction=showfull&id=1505495519