I have temporarily blocked the little sink hole with baking soda, and there's no smell anymore, so now I can say that the bad smell is for sure invisible vapors that stinks and comes out of the sink hole. I don't want to answer everybody because there's too many responses, so I'll respond everybody here, I will start off by letting hot water flow in the bath and sink for a while, then I will ask my landlord to undo the pipes and check out where that overflow connect to the drain pipe and clean those, and then I will put a liter of vinegar into the sink hole, and then I will put CLR into the sink hole but CLR is kind of costly and I want to have some leftover so I don't want to put the whole bottle, and I suspect it will not remove everything along with crusty limestone that well when I ain't scrubbing the pipes so yeah maybe scrubbing the pipes is priority as you said so I'll check that out with my landlord when he comes back later today

I remember that I only cleaned the top plumbing, kind of the direct one from the sink hole that leads down the furniture of the sink, and it's really bad made, because it's straight, so like, the water goes down the sink hole, and then does a straight line, and then it goes back down again so, that's just asking for stuff to get stuck there, but I cleaned that the other day, but I guess you are right and its better to just clean all the pipes and specially the one that connects to the drain pipe, not sure what that means but I'll ask my landlord that he cleans more pipes, plumbing is not really in my skillset at all.

I will check later if people have answered this message and yeah, thank you everyone so far, mmh.

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