Not seen this mentioned elsewhere, in case others might find it useful, here's the way I have found to transfer routes onto a Garmin.
I've used Ridewithgps to create the route (the mob app works great), all it needs is for you to get the GPX to your phone. Then, you will need an OTG cable for the correct version of USB connector on your phone and ES File Explorer app on your phone.
Plug in the OTG, the garmin into the OTG USB female connector, let the Garmin start (it should go into USB device mode) and Robert is your mother's brother.
Use ES File Explorer to copy and paste the GPX from your phone to the right Garmin GPX folder (the device will appear as "usbnnnn" on ESFE). first time you use it you may have to familiarise yourself with ESFE. No need for root.
Job done.
Background info:
> Tried with 2 Nexus phones running Android 6, 1 x Micro USB, 1 x USB C. Not all OTG cables are the same quality, take particular care how bulky the USB end is when buying, some may not fit your phone in its case (if you have one, some just don't wurk
> I have ES File Explorer Pro - it SHOULD work with the free version, but the ESFE is the best value $3 I've spent on my phone
> Tried with 2 x Oregon devices, 1 x Edge 350
> I used RidewithGPS pro (which I probably won't keep up, but have bought for one month) which made it easy to download - how you get your GPX is immaterial
Be interesting to know if it is universally as easy or if the combination of native android etc makes it so.