Hi all, I have recently moved into a property and have noticed some of the bathroom shower tiles were loose, so I pulled them off and am enquiring about the best way to adhere them back on so water doesn't leak in the gaps. The tiles are adhered to a concrete slab which forms the base of the step in shower. I am not sure if I should waterproof the area or reapply the tiles and grout only. Any advice would be appreciated !

