All of my runs on Garmin use routes and sync just fine.
Step One: Create a route and send it to your watch
Step Two: Initiate a run activity. Do NOT press start (the top right button)
Step Three: Load a route. Press the middle left button. Select route with the top right button. (If it’s not the first choice, use the left buttons to go through the menu options.) Find the course you want to run by scrolling through the courses on your watch. Select it with the top right button. Then press the top right button AGAIN selecting Do Course. (You can also do it in reverse, see it on a map, delete it, etc.)
Step Four: Wait for the top of the map to turn green, indicating that you’ve got a GPS lock.
Step Five: NOW start the run by pressing the top right button. It will come across as a run and show up on City Strides just fine.