It doesn't seem right that newly transplanted plants would croak so quickly moving into s/h. My hunch is that you are watering too much. The media looks dry on top long before it dries out at root level.
Did you soak the pellets before using them? In general, instructions say to use well soaked pellets and then not to keep water in the reservoir for several weeks until the plants establish themselves and root growth starts. When I do this, I flush the pot well about once a week to keep moisture in there, but don't use the reservoir for several weeks.
Don't know about your light conditions. If I understand what you mean, most phrags would like a bit more light. And they would want to stay moist--more than most orchids. Was it a phrag you lost?
There are some who might disagree, but if you are in the northern hemisphere, going into winter isn't the best time to establish plants in s/h. Tends to be chilly and dark and dry. I have some paphs and phrags that I am going to overwinter in traditional media and put into s/h about next June when growth gets ramped up again.