Underfloor heating requires flexible adhesives for fixing tiles. Moreover, you have chosen large-format tiles. Hence, in this case, the proper adhesives will be AK 516 and AK 517. Those are liquefied adhesives. Thanks to their semi-liquid consistency, the adhesives eliminate air gaps under tiles. The whole surface of the tile is attached to the floor, which is extremely important with underfloor heating. Another advantage is the fact that those adhesives are quickly-bonding ones, and binding can be commenced 6 hours after the tiles have been laid.