I had some struggles downloading a route from Strava to Garmin with Connect being down.
Hopefully my workaround can help someone else.
From Strava download your file as a GPX.
There are a couple converters from GPX to FIT, only file extension mine will read.
Connect Device to the computer by USB.
In file explorer of device there is a folder location for “Courses”.
Save the .FIT file to that folder.
It should now have the route on your device.
I have only tried this with one route and haven’t ridden it, so it might not be perfect.