Hi Jenny :-)
The Fullscreen Slider should work. Probably you missed to check a box in the settings connected to Custom phone layouts or something. Best is to play around with it. You can also activate phone layouts within a project. It is a little phone icon on the top right of the Gridder.
It is difficult to guess without looking inside of your Wordpress.
But I am happy you solved it via the carousel.
Many wishes!
Marius