Best choice might be, Ardex EG 8 PLUS epoxy. It will work as an external adhesive & a grout. Its so much easier to work with than previous epoxies but its expensive.
Otherwise any cementitious bagged adhesive powders will, nowadays, most likely work for internal & external - use BAL, Mapei etc. top brands only.
Use the flat, back of the trowel to skim the board surface then comb the board with a 6mm notched trowel.
Then comb the back of the tile.
It fills in and gives wriggle room.
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