One of the most important things to do as part of overseas travel preparation is turning off roaming and mobile data. Have you ever in the situation when you received high mobile data bill after the overseas trip and you ‘thought’ you never used internet (never opened browser, didn’t open WhatsApp, etc). It is because smart phone has some background processes that we knowingly and unknowing set them to run.
The best solution is to turn off the roaming and mobile data completely. This is how to do it. Please note that you will still use Wi-Fi connection (ie. connect to the pocket wi-fi)
Iphone (iOS 11.4)
Go to Settings -> Mobile and you will see the following screen.
Slide the green slider to turn off the Mobile Data completely. Green means the Mobile Data is on. Grey means it is off.
Tap on the Mobile Data Options and make sure Data Roaming is greyed like the following screenshot. That is it. You are done!
Go to Settings -> Wireless & networks as shown in the following screen.
Uncheck both Data enabled and Data roaming.
Hope this helps.