coverage in korea

Assuming you have a modern phone and a current postpaid plan, you should have great international roaming service in S. Korea. Depending on your plan an needs you may want to pick up an international data pass. You can do this via the T-Mobile app after you arrive.

https://t-mobile.com/support/coverage/international-roaming-checklist

https://www.t-mobile.com/support/coverage/international-roaming-services

https://www.t-mobile.com/coverage/roaming?country=South%20Korea