There are two types of routers. In the Netherlands the shafts are 6 and 8 mm, in the US and UK they are 1/4 inch (and maybe other sizes but all based on inches). In short: it will never fit. Probably buuying a 1/4" shaft will not solve your problem as the slot cutter will not fit on it. So buy a (second hand) router from Marktplaats (I assume you are Dutch based on arcadewinkel.nl).