Cannot find an add button in the wi-fi section or connect to a hidden network I just see all my neighbours wifi...

Of course a hidden network will not show, that's the whole point of being hidden, isn't it?

Sadly you didn't provide the most basic information (Distribution, exact Plasma version, where you tried to add the network), but try this:
System Settings -> Network -> Connections: click on the + sign at the bottom, select WiFi connection and enter your credentials.