Whoops, sorry about that, I grabbed the wrong bookmark. I meant to refer to https://garmin.bbbike.org/. Whilst not specific to the Extrx, DCRainmaker has guidance on using the website/installing the maps at https://www.dcrainmaker.com/2019/08/...rmin-edge.html. Ray links to https://garmin.openstreetmap.nl/, which is no longer active but refers you to bbike.org.

Hopefully, that works.