I don’t have a smart tv but I’m guessing their confusion comes from only being able to turn it on while watching a video that has audio descriptions. But when you turn it on from within one video it turns it on for your user on all devices.
Lost In Space has audio descriptions and should be easy to find since a new season just came out.
For an accessible description of how to turn on audio descriptions on most devices
$1 Or go to acb.org and Netflix will be in the main menu.