OK, the drain cleaner worked short term but the smell has come back. This suggests to me that you have no traps in your drains. These isolate your drains from the main drain and "trap" a quantity of water which allows water to drain past but smells (and bugs) are prevented from coming back up. It's the same principal as a toilet bowl and if you have ever had the misfortune to let one of these go dry you will recognise the smell as being the one you are experiencing now. You need to get a plumber or builder to fit traps. They only cost a few euros and it should be a fairly easy job and with any luck should not damage the surrounding tiles. I'm not a plumber but, from what you have said, this sounds to me like the solution.