More than once now, when my iPhone 12 mini and my wife's iPhone 12 pro leave a location where we'd been with w-fi and wi-fi calling, the phone detects a LTE or 5G signal but doesn't have data available. I note it particularly when I want to use Google Maps for directions.

I've pulled onto the roadside, shutdown and restarted; all works fine.

How do I avoid this issue?

Thanks!