i had trouble tiling a wooden plywood bath panel last year, i used some unibond flexi stuff after priming the wood, but it was no good GRRRRR so i started over, i went to the builders merchants got a fresh sheet of thick external grade ply and started over. i laid the panel flat on the floor i marked it carefully then stuck the tiles on with BAL Fast flex 2 part mix (white), and left it to set over night in the morning i lifed the panel into position drilled and screwed it in place then grouted it with flexible wall
tile grout, sounds like over kill i know,and pricey. but that adhesive is practically bomb proof and very flexible.