Five settings, all of them in one menu, all of them faster to fix at home than at an airport
The Five
- Install the eSIM over Wi-Fi and give the line a name you will recognise
- Data roaming ON for the travel eSIM — without it, nothing connects
- Data roaming OFF for your home SIM — without this, your operator bills you
- Mobile data set to the travel eSIM, so the phone uses the plan you paid for
- Calls and messages left on your home line, so your own number still works
On an iPhone, also turn off Allow Mobile Data Switching (Settings › Mobile Data › Mobile Data Options). It lets the phone fall back to the other line when the first has poor signal — sensible at home, and a roaming bill abroad.
Worth Doing at the Same Time
- Set photo backup to upload over Wi-Fi only — the largest data user most people never notice
- Turn off automatic app and system updates over mobile data
- Download offline maps for where you are going
- Check any app that streams video and lower its quality for the trip
Leave the travel line switched off until you land. Everything above stays configured; the line simply waits.
A minute checking these at home saves the specific bad half-hour where you are trying to read settings menus in an unfamiliar airport with no connection to search for help
