I have been told alway use silicon to bed them down on. It's flexible and bonds to the tray and plywood. BUT......... Manufactures state sand and cement. Don't want to upset the warranty. But surely it's better to use silicon
If it has failed on Tile adhesive it would of failed on cement . We have had failures on Silicon random crack in tray . We stick lots of trays with flexi Tile adhesive much like tiling you have to make sure you have good coverage