Hi, are you still struggling with this?
I was wondering what you actually want the outcome to be?
Do you want the door icon to always be visible on the screen even when you are not looking at it- which I am not sure why you would want that or how?
Or do you want the icon to be visible on the door when having the door in frame - in which case I would use a World Space Canvas linked to the door object and put the icon on that (any maybe change its scale in regards to how close the player is).