If you are having trouble getting roaming to work on your service while you're overseas, you might need to manually select your carrier network on your device.
How to Manually Select Carrier Network on iPhone
1. Head over to “Settings” from the home screen of your iPhone.
2. In the settings menu, scroll down and tap on “Cellular” to proceed.
3. Next, choose the “Network Selection” option located below the Personal Hotspot.
4. Here, you’ll see that network selection is set to Automatic. Tap once on the toggle to change it to manual.
5. You’ll need to wait for a few seconds for all the available networks to show up. Now, simply select your carrier or partnered network provider for international roaming and you’re good to go.
How to Manually Select Carrier Network on Android/Samsung.