shower tiles are coming off

[JK2]

Some shower tiles have come unstuck and I want to reattach them. They are attached directly on a concrete wall.  i think I need to clean the wall, make sure it's dry, stick the tiles back on, grout?, and put some.sealant on. Does that's sound right and what are product suggestions? Thanks JK2

[iinsaayn]

I have the exact problem. I'm afraid to touch some of the edge tiles in case they fall off! Your thoughts seem accurate, but I'm interested in the suggestions too. I'm wondering if there needs to be waterproof paint to the concrete too?