How to upload and Access a .gpx router or course from your mobile phone to your garmin watch when you're outdoors in the wild, offline (no GSM or very bad connection).

I followed the Garmin Explore instruction below. So fare it works. 
The main issue I can't finde the Cours Menu under Navigation on my Watch how to I enable it. 
For me it was the main reason I upgraded from Fenix 3 to the Enduro 3. As I use the Watch to navigate in a withe out on the glacier to find my way out or access the mountain hut. 
Be save to not step out of track. 

Thread from Fenix 7:
It is possible through te garmin Explore app. 

At home you need to install and let the app automatically login with your account into the garmin explore app, make sure you see your watch and are able to synchronize it via bluetooth. no need to download maps etc.
Then once on the mountain, with or without GSM, turn on Bluetooth, go to the file manager on your phone and open the .gpx file. Then the file manager will ask you which app you want to use to open it. Choose garmin explore. (It may happen that you need to open the file using 'open with' option. This trick works with all files and all apps in Android. Don't know what happen in a classy Iphone)
Garmin Explore app will open the file and make some questions and will show you the route....(on top of a map if you downloaded maps on garmin explore or in a blank screen if not) Then go to the watch icon at the lower part of the app and choose your device (or Garmin watch) to sync... and that's it. Your .gpx is available in your watch.... all of it totally offline. Neat!!
This is a very important safety feature for me, because once everything goes wrong,  the bigger screen on your mobile can help you understand better how to get out of a messy route down to civilization. The  you van create a route and use it to get out of the mess. And I have always use oruxmaps app and  Brouter with offline detailed maps to do it, sending the 'escape' route to my phone in order to follow it and to save the battery of my phone for safety reasons.
There are other two ways of doing it but those are more cumbersome...
1)install and pay DWMAP from connectIQ and follow the same procedure but opening at dwmap. By the way DWMAP was the only way to follow gpx route in the good old days with the first garmin smartwatches... and it mY work for you if you have an old garmin watch. It works seamlessly and awesomely really good. I used it for 7 years until I just bought this fenix 7 and was shocked to discover that I could do less than with the old DWMAP until I discovered this trick in garmin explore.
2)or carry a USB otg cable with a Garmin cable to connect your mobile to your device, and copy paste the .gpx file into the gain folder of your device using file manager. This does not work in my android (bussiness phone) so it not work also with your phone or your classy iphone