I have started international roaming to Korea, and I will call and send text messages as soon as I get off the plane. Why is there no signal when I leave the customs?

According to the situation you described, the international roaming function has been turned on. When you can't get a signal by using iPhone overseas, you can manually select the network. Click settings > operator manual selection.

After leaving the country, you can't automatically log in to the local operator's network with iPhone. First, confirm the opening of international roaming service, and then solve it through "manual network selection": enter Settings from the mobile phone panel >; Operator. At this time, the mobile phone will search the currently available networks and display a list of roaming network operators, from which the most suitable network can be selected to use the international roaming service.