The travel eSIM. On an iPhone: Settings → Cellular → Cellular Data → choose the travel line. On Android the same choice is under the SIMs screen, as the mobile data preference.
This is the setting that stops your home carrier billing you for roaming. Everything else can be right and this one wrong, and you will still get a bill.
Leave your home line switched on if you want calls and texts on your own number. It should not be the one carrying data.