i put it to the place where the screwdriver-tip is pointing to on the first pic. It worked for a couple of weeks, then the mic-module lost contact again, probably due to squeezing down of the rubber-material, which was quite ductile. I replaced it now, using a little piece of an old bicycle inner tube (picture 2), which should be quite non-deformable. After fixing the screws tightly, mic was working again. I hope, it will last longer this time.
Good luck
Fairfish
Additional Information from 9th June 2020: Mic ist still working.
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OT: On the one hand, I am amazed that I can fix my phone with just a bit of tape. On the other, I wonder if I would run into this problem with a regular phone. Guess I will never know.