What is this connector (and the eventual pliers used for it) that serves for Wago 256 terminals.
Those look like ordinary insulated crimp ferrules. A search for "ferrule" on Mouser, for example, returns lots of results like this. The crimp tool that you select will typically depend on a number of factors, including the brand of ferrule that you select and whether or not you want a hand crimper or production-quality tooling. Any decent-sized supplier should be able to help you identify the tooling options to match the ferrule that you select for your application.
There's nothing magic to match a Wago 256, as it is an ordinary terminal block with a cross sectional area range of 0.08 - 2.5 mm² per the datasheet. Any ferrule that meets this requirement should work just fine.