HomematicIP model Hmip-FSM connection of a smart switch to control an LED transformer

Good morning

I would like to ask for help to connect correctly to the installation - smart switch module HomematicIP model HmIP-FSM .
The device looks like this: .
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The connection diagram provided by the manufacturer looks like this:


I have LED strip illumination of the stair treads. At the stage of finishing the flat, I did not envisage making stair switches on the ground and ground floor so that I could switch these stair lights on at the lower level of the stairs and off at the first floor. So that I don't have to rip up walls and add cables, I came up with the idea of setting up a smart switch so that I can switch the staircase lighting on the ground or first floor remotely.

The mains wiring in this area currently looks like this: .
Under the stairs I have a room in which there is a simon10 double wall switch - a,b, with which with "a" I switch on the LED transformer and thus illuminate the stairs with led strips, and with "b" I switch on the wall lamp to illuminate the room under the stairs.

Diagram :
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Before the transformer I would like to connect a HmIP-FSM smart switch so that it can remotely switch the LED transformer on/off.
My idea of how to connect this below. The way the switch is connected to the original installation is shown in the pink rectangle. However, with this connection, the switch does not work - as if the smart switch has no current (because the LED indicating the device is working does not light up).

Diagram :
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The LED transformer is model POS200-12-C2
something like this:
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Maybe you have an idea what I am doing wrong and how to connect it to make it work? .
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AI: Can you describe exactly how you connected the HmIP-FSM switch to the existing installation? Details of the cable connections would be very helpful. .
According to the diagram attached to the post
AI: What are the power supply specifications for the LED transformer and are they compatible with the HmIP-FSM switch requirements? .
according to the photo/description attached to the post

>>21325916 .
Thanks for the hint, but if the problem was the lack of pressing the "a" key on the wall switch, that would mean I'm a d3bile wasting my time writing a post and making diagrams and posting them on the forum and the time of forum members reading this material.

Obviously despite switch 'a' being on, the switch does not work. It appears that from a purely electrical point of view the connection is correct. I tried to test connect it in an analogous way but to a wall lamp, which is controlled by the "b" key - to exclude the incompatibility of the current characteristics of the switch with the transformer, but with an analogous way of connection but to a wall lamp - the smart switch does not work either.

So I assume that maybe the smart switch itself is faulty/defective. I will try to arrange for a second one to replace it. Unless the problem nevertheless lies in an incorrect connection. I am particularly doubtful whether I have correctly connected/spliced the neutral wire which is connected from the vago cube to both the smart switch and the tranformer - here in particular I would like to ask for confirmation