It seems Wi-Fi was available on the train but the app hadn’t/wouldn’t download where they spoke to the RPI as there was no Wi-Fi at that particular part of the station; only 4G or whatever was available which was insufficient despite having plenty of data remaining.
For what little it’s worth, this probably isn’t a question of 4G “being insufficient despite having plenty of data” - every smartphone I’ve had in recent years (personal iPhones and work Microsoft / Samsung) default to only updating apps over wifi