The joints can only be smoothed mechanically; with a flat screwdriver, a knife, i.e. clean out the tile joints and grout them again.
Grout the tiles after 3 days after laying. Grout a small area of the tile with a soft rubber spatula. After 10 minutes, run a dry finger horizontally and vertically over the seams and then carefully go into the corners of the tiles (overlapping only +/ leveling with straight lines). So grout the entire tile. The joints must be smoothed immediately after grouting the tiles, because even after an hour it is difficult to smooth the joints. Remove the remains of the grout with a dry, soft cloth, making sure that no small grains of sand remain in the joints.
Then wash the tile. Try it, but be careful in the corners of the tiles.