Arctic Char can be caught everywhere in East Greenland. They are typically caught in river estuaries or at the point between the lake and river. As a rule of thumb, char are found where the concentration of food is highest, where the water is deep enough to cover their dorsal fins and relatively still.
Take, for example, Adventure Camp Greenland as your starting point, where guides can help you find the good fishing spots. The season runs from mid-July to late August.