Roaming allows a device to connect to foreign mobile data networks. This option is usually turned off as connecting to foreign networks can cause high costs.

Android 9
Step 1:  Open the  settings
Step 2:  Tap on  Network & Internet
Step 3:  Tap on  Mobile network
Step 4:  Activate or deactivate  Roaming
Step 5:  Confirm with  OK
  1. Open the settings
  2. Tap on Network & Internet
  3. Tap on Mobile network
  4. Activate or deactivate Roaming
  5. Confirm with OK

Android Instructions
Read more