Routes go through places phones don't. The short version: the phone app is built for that and the web app isn't. If you expect to lose signal, drive from the app.

On the phone app

The app assumes you'll go offline and tells you when you have. A banner appears — "You are offline — some features are unavailable" — and stays up while you're out of coverage.

Anything that needs the server is then blocked deliberately, with "This action requires an internet connection" rather than a failure. That covers signing in, creating and changing routes, editing stops, all three import methods, sharing, and purchases. You get Retry to try again once you think you're back; if you aren't, it says so — "Still offline — check your connection".

What still works with no signal:

Looking at a route you've already opened, including the stop list and their order.
Your saved stops, which the app keeps its own copy of.
GPS. Your position still works — that's the satellites, not the network.

What doesn't:

Anything that changes data, which is most of the app.
Turning a location into a street address. GPS gives coordinates; converting those to an address needs the network. So a stop you capture in a dead zone is stored as a point with no address until you're back. See A stop shows an error.
Map tiles you haven't already loaded.

On the web app

The web app has no offline handling, and this is worth knowing before you rely on it.

There's no banner. Nothing on the page changes when your connection drops, so the first sign of trouble is an action not working.

And when an action fails, it fails silently. Press Create on a new route with no connection and the button turns into a spinner and simply stays there — no error message, no toast, no timeout that hands you back control. Nothing was saved, but nothing tells you that either. The same shape of failure applies to other actions that need the server.

If a button in the web app has been spinning for more than a few seconds, check your connection, then reload the page. Reloading is what gets you out of it. Anything you'd filled in but not successfully saved is gone, so re-enter it once you're back online.

The one part of the web app that does handle it is saved stops, which tells you plainly — "Cannot save while offline" — instead of hanging.

Which to use where

Situation	Use
Planning tomorrow's route at a desk	Either — the web app is roomier for building and editing
Driving, with coverage you trust	Either
Driving anywhere rural, underground, or across a border	The phone app
Somewhere you know you'll have no signal	The phone app, and open the route before you go