I wonder if this could be Finders Keepers by Emily Rodda? I haven't read it, and Wikipedia doesn't have any information, but I did see the TV adaptation which matches a couple of points:
The teenage protagonist is invited onto a TV show on a channel that doesn't exist.
There's a barrier between worlds which people on one side can touch
The protagonist crosses the barrier by means of a TV set
The game is about finding items that were lost.