Don't know much about iPhone apps, radio/camera units, and so on, but you could use a standalone Garmin GPS for navigation and add lifetime maps and traffic to it. They're not tied to cell towers for navigational function.
2018 (2017 Sprinter Cab Chassis) Navion24V + 2016 Wrangler JKU (sold @ ????) - 2016 Sunstar 26HE (sold @ 4600 miles) - 2002 Roadtrek C190P (sold @ 315,000kms)