First — no, it won't cost you extra
"Roaming" sounds expensive, but with a Roamly eSIM it isn't: your plan price is all you ever pay. Data Roaming is just the switch that lets your phone use an overseas network — your Roamly plan is the overseas network deal. Leaving it off doesn't save you money; it just keeps you offline.
The only thing to watch is your personal SIM — leave its Data Roaming off, and your home carrier can't charge you a cent. You only turn roaming on for the Roamly line.
On iPhone
Open Settings → Mobile Data (called Cellular in some regions).
Tap your Roamly eSIM under SIMs — it's the one you named when installing (often "Travel").
Make sure Turn On This Line is enabled.
Toggle Data Roaming on.
Go back to Mobile Data → Mobile Data at the top, and select your Roamly eSIM as the line to use for data.
On Android
Menus vary a little by brand, but the path is always close to:
Open Settings → Network & internet → SIMs (on Samsung: Settings → Connections → SIM manager).
Select your Roamly eSIM.
Turn the SIM on, and toggle Data Roaming on.
Set the Roamly eSIM as your mobile data SIM.
Still not connecting?
Give it a quick once-over: the Roamly eSIM is set as your data line, Data Roaming is on, then flick Airplane Mode on and off (or restart your phone) to force it to find the local network. Most phones connect within a minute of landing on the right settings.
Still having issues? Visit our troubleshooting collection.