There is a power point in the cupboard. The wired connection would be from the router to the Mesh 'base' station, which would be in the same cupboard. This would then, technically, feed the other Mesh devices upstairs via the power lines. This is what I have at the moment:
https://amzn.eu/d/9LSFUDII'd have to get an add-on 'station', as I've only got two at the moment and one of them would need to be downstairs with the router, but that shouldn't be an issue.
The only thing is, if it didn't work well, what would it cost to have them come back and reinstall it by drilling holes?