OnePlus
Enable or Disable Data Roaming auf OnePlus
Data roaming is the setting that lets your OnePlus use mobile data through a foreign network operator when your own provider has no coverage in that country. Without it, your SIM can still make calls in some regions, but internet access over mobile data stays blocked until you connect to WiFi.
Every SIM slot on the phone has its own roaming switch, so a dual-SIM setup needs the toggle enabled separately for the card you plan to use abroad. The setting sits inside the same menu that controls the rest of your SIM and network behavior, not in a separate roaming app.
To change it, open Open the Settings ➔ Tap on Wi-Fi & network ➔ Tap on SIM & network ➔ Choose a SIM card ➔ Enable or disable Roaming ➔ Confirm with OK.
Turning roaming on does not automatically add extra cost by itself, but any data used while connected to a foreign network is usually billed at your provider's roaming rate unless you have a travel or EU data plan. If you only need messaging apps or maps for short trips, WiFi at hotels or cafes is often the cheaper fix instead of leaving roaming active the entire stay.
Once you are back on your home network, switching roaming off again prevents the phone from reconnecting through a foreign tower by mistake, for example near a border.