When you go abroad, you are at a loss whether to use the current (domestic carrier) SIM card for smartphone communication or the prepaid SIM card at your travel destination. Since you just purchased a device and the SIM lock is on, you can not use the prepaid type. Should.
If you want to communicate with a SIM card currently in use overseas, you can turn on the "Roaming" switch in "Settings"-> "Network and Internet"-> "Mobile network". There is also a way of thinking that it should be turned off. However, it seems to be forgotten by the busyness after landing, and it is a little troublesome.
Therefore, we would like to consider overseas data communications flat-rate services provided by communications carriers. NTT Docomo offers “Packet Pack Overseas Option”, KDDI (au) “Global Data Flat-rate”, Softbank “Unlimited Packet Overseas”, and offers the same charge regardless of how much you use for 24 hours. (Some countries / regions are not available), so make sure to apply in advance using the dedicated app.
These flat-rate services do not incur communication charges even if the roaming switch is turned on unless you start the operation. So take off with the roaming switch ON, land at the destination airport and release the airplane mode when you are allowed to use your mobile phone, tap the start button on the dedicated application (the communication carrier has partnered You will be able to use a local carrier's mobile line at a fixed price.
Conversely, if you do not use overseas data communications flat-rate services, be sure to turn off the roaming switch. This is because it may automatically connect to the local communication line. Android OS may automatically perform various processes such as notifications and app updates, so be careful.