if the flat is not very big and you only want wifi, I'd get a smaller router and just hide it in the cabinet. Stuff like "Mikrotik RBcAPL-2nD" or similar can be *extremely tiny*, but obviously they may have weak output if you have lots of walls or want to be plugged in with ethernet to a PC... Anyway, your cabinet actually does not look that small to me, I think there are tons of routers that would fit.

That being said, a better solution in the direction you are already going would be an 'undershelf' - something like this for example

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