ᐅ Issue with connecting sink to wastewater pipe/drain

Hello everyone,

As a non-professional in plumbing, I have a question whether anyone is familiar with this or knows a solution for the following problem.

I recently bought new bathroom furniture including a sink at a furniture store, and these items were delivered by their company. The in-house technicians installed everything, including the water connection for the sink. Unfortunately, the technician/installer had significant difficulties attaching the drain to the sink because he couldn’t properly fit the parts together and didn’t know how to do it – supposedly it’s a new type of drain.

Now, when the temperature rises, there is a bad smell coming from the sink drain. The sink has NO overflow, and as far as I can tell, there is no proper trap installed. (Previously, we had a different sink with a trap, and there were no issues with unpleasant odors.) You can also clearly hear noises coming from the pipe when a neighbor one floor above drains water from their sink and it rushes through the pipe.

I have the impression that the connection between the sink and the drain pipe inside the wall is not sealed by water (as in a trap), allowing noises from the pipe to travel through the connection pipe from the sink drain into the bathroom.

I’m attaching two pictures to at least illustrate the situation from the outside.

Can anyone tell me what is missing or what might have been installed incorrectly, if this can be determined from the pictures at all?

Thanks and best regards,
Gregor