Need to fix tile that has fallen off
I would recommend heavy duty constructive adhesive followed by grout colored caulk that best matche the grout around the tiles https://www.google.com/search?q=grout+colored+caulk

to minimize cost and work I would risk using the caulk as glue


try to remove the old adhesive
clean back of tile and wall well
put a lot of construction adhesive on the back of the tile (risky optional: replace construction adhesive with caulk)
put tile back on the wall and hold in place with masking/painter tape
1  hour later remove tape
caulk around the tile and clean any thing that doesn't look neat with your fingers