You can set the default navigation app in Options > Routing > Navigate with. If you have an account and are logged in, that will choice will be remembered too.
Hm… so I actually just tried this. I made sure “Google Maps” was selected in options, and on my iphone when trying to navigate from the route, it’s still asking me if I want to install App Maps (which I removed from my phone).
Maybe your phone uses different browser apps. You may be using Chrome as your favorite web browser from the home screen, open the RouteXL website and set the options correctly. But when you tap a route link in an e-mail or Whatsapp message, the phone may use Safari as its default browser to open RouteXL and different options are used.
We’ll work on a solution for this, but you may want to check the options in Safari and change them before navigating.