Data roaming is the phone setting that allows mobile data to work on networks other than the one that issued your SIM. It exists as a per-line toggle in your settings, and nothing about using data abroad works until the line in question has it switched on.
The toggle has a scary reputation because, on a home SIM with no travel bundle, enabling it opens the door to expensive roaming charges. That fear leads to the single most common travel eSIM mistake: leaving data roaming off on the eSIM line. Travel eSIMs connect through partner networks abroad, so the phone treats the connection as roaming; with the toggle off, the eSIM installs fine but never passes any data.
The key point is that the toggle controls permission, not price. What you pay is decided by whoever bills the line, not by the switch itself. A prepaid travel eSIM cannot charge you beyond what you already paid, so enabling data roaming on it carries no billing risk; the danger only exists on lines with open-ended billing. That is why avoiding roaming bill shock comes down to which line the toggle is on, not whether it is on at all.
The safe travel setup takes seconds: turn data roaming on for the travel eSIM, off for your home SIM, and set the eSIM as your mobile data line. All three steps appear in our installation and activation guide. If data still fails afterwards, the next suspects are network selection and the APN, both covered in the troubleshooting guide.
A concrete example: a traveller lands in Thailand, installs an eSIM at the gate and gets no internet. Settings show the eSIM active but data roaming off for that line. One toggle later, maps, messages and translation apps all spring to life, and nothing extra is ever billed because the plan was paid for before the flight.