Most of the maps on this site do not support turn by turn navigation. You can find routable maps by going here and check the "routable" box under "Search by Map Features"
https://www.gpsfiledepot.com/maps/You will only get 13 results from that search. So, are you using a map that actually supports routing? If not, it won't work properly. However, the GPS should not not crash. If the map does support routing, you could try contacting the author for troubleshooting.
Otherwise, I'd say you are out of luck, since Garmin will not give you any support for the maps from this site.