Android
Turn Data Roaming On or Off auf Android
Data roaming lets your phone connect to a foreign carrier's network when you are outside your own provider's coverage area. Without it, mobile data stops working the moment you cross a border, even if your phone still shows signal bars from a partner network.
Most carriers ship this setting turned off by default. That protects you from unexpected charges, since foreign networks often bill data at a much higher rate than your home plan. Turning it on makes sense when your plan includes roaming data or you need internet access while traveling and cannot rely on Wi-Fi.
On a dual SIM device, roaming is set per SIM card, so you can allow it on a travel SIM while keeping it disabled on your main line.
To change it: Settings ➔ Network & internet ➔ SIMs ➔ Choose a SIM card (Dual SIM) ➔ Enable or disable Roaming ➔ Tap on OK.
If you only need occasional foreign access, leave it off and switch it on right before you need data abroad. Turning it off again once you are back on your home network avoids any accidental charges from lingering background connections.