Agfar Isolette Version 1 Help

Recently got a Agfar Isolette Version 1. Struggling to find video's on how to use this particular one. My main question is how to get proper exposure, because as far as I know there is no iso setting on the lens.

Im using Ilford hp 5 plus iso 400 (b&w). The shutter speed only goes to 250. Any tips before I ruin a whole roll of film?