In order to be able to use the mobile data abroad or with a foreign provider network, we have to activate the roaming function on the OnePlus smartphone, which only then allows a connection to the Internet, what can lead to increased costs.
Depending on the provider, additional APN settings for roaming have to be configured.

Note: If we are in another EU country, we can use EU roaming, which means that we can use the Internet volume as if we were in our home country.
We still recommend that you check with your mobile phone provider beforehand to avoid unexpected costs.

Android 10 | OxygenOS 10.0
Data roaming -  Open the  Settings
Data roaming -  Tap on  Wi-Fi & network
Data roaming -  Tap on  SIM & network
Data roaming -  Choose a  SIM card
Data roaming -  Enable or disable  Roaming
Data roaming -  Confirm with  OK
  1. Open the Settings
  2. Tap on Wi-Fi & network
  3. Tap on SIM & network
  4. Choose a SIM card
  5. Enable or disable Roaming
  6. Confirm with OK

OnePlus Instructions