Bathtub siphon replacement, one-piece set

Hello,

We have a clog and after I cleaned everything, I discovered that the siphon is clogged. It is disappointing that the part cannot be replaced (Viega, but is it all one piece?) and the entire assembly has to be replaced.

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There is an inspection opening. But I am not sure how to remove the overflow? I don’t see a screw or anything similar anywhere. Of course, the new line for the overflow has to be somehow brought up.

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The problem is that we are currently installing new tiles in the bathroom and want to close the inspection opening soon. The earliest appointment for a plumber is in one month. That is fine (maybe we will leave the last piece of tiles open for the inspection opening), but if it is not too difficult to do a replacement myself, I would do it myself.

Thank you.