The WetBase should be fixed to the under boarding platform securely  using a good quality flexible tile adhesive, bought in powder form. The reason is buildings potentially move, albeit only marginally,  as the ground below them shifts according to the time of year and without that flexibility the tiles above the WetBase potentially will move too leading to cracks in the grout or even the tiles themselves.
Under no circumstances should any of the following fixings be used.
Solvent based adhesives—they will damage the composition of the WetBase Screws—they will damage the WetBase and its the waterproof properties
Ready mixed adhesive—only adhesive mixed from powder should be used. Ready mixed adhesive contains too much water, and additives that extend its shelf life, that can delay or even prevent adequate drying out and therefore adhesion.
Suitable brands of powder adhesive include Weber (Floor Flex or Weberset SPF), Mapei (ProFlex or Keraflex), Granfix, and Larson Proflex.