Can you clarify why you want to view the unlit mode on the Quest? Is this just for debugging purposes or is it a feature of your game?
It may be that the “viewmode” command only exists when running in the editor (I’m not sure.). The good news is that it’s easy to recreate that view by creating a simple post process material. Here’s what the implementation would look like.
You would apply this post process material to your scene using a Post Process Volume and adding it to the “Blendables” section. See the UE docs for more details.