There's a water mark down the brickwork in your post #5 first pic. Is that significant?
Is the black timber in that pic a new addition? Water from the overflow pipe or gutter above it could get trapped behind the timber.
Nor do I. I suspect that those who have invested heavily in the gizmos swear by them (the Emperor's New Clothes syndrome), whereas others swear at them. :giggle:
As that circular part will be covered by the other part you could perhaps get the drill out and make a couple of fixing holes in it, then use stainless screws to fix the part to the wall? Just don't put screws through the plumbing/wiring !
I misinterpreted your pics.
That video apparently shows the big circular part fixed to the wall with what looks like 4 (nylon?) screws, but I don't see fixing holes in the part you have?
Are you sure that back plate is fitted correctly? Looks to me as though it needs reversing. The buttons should protrude through that plate and through the front cover. The edges of the back plate would be gripped by the castellated edges of the cover.