Garmin should be able to help but will not give you any advice about OSM maps.
Download this map: osm_generic_new_gmapsupp.zip from this link
http://osm.pleiades.uni-wuppertal.de/garmin/generic_new/07-06-2018/6405a2962a33e39de7212c36893f917a/
Unpack it and rename gmapsupp.img to iceland.img
Copy this file to the map directory on your device.