Hi @Janny111,
Realistically, the only way to fix this problem is to remove the sunken tiles and reinstall them using more tile adhesive to raise their level slightly. If the dip is significant, as in 10-15mm you may need to use a self-levelling compound such as Lanko 173 Floor Leveller before reinstalling the tiles.
As the tiles are in your kitchen, it is unlikely that there will be a waterproof membrane beneath. This means you can lift and then reinstall them individually.
Did you have any spare tiles that you could use?
If not, you would need to be very careful removing the tiles. Start by removing the grout with a grout saw, then gently lever the tile up with a pry bar being as careful as possible to not break the tile.
Once removed, you will need to clean up the tiles and the substrate and then reinstall them. You can use this Ezitile DIY Leveling Starter Kit to ensure you get things nice and level.
Let me know what you think and if you have any further questions, please don't hesitate to ask.
Jacob
