I activated my Ubigi plan hours prior to departure. During my flight, I disabled my cell phone provider SIM and activated the Ubigi esim.

As soon as my flight landed in ICN, turn off airplane mode and my phone worked instantly. At the top of my phone, it says SKTelecom/Ubigi

Now we shall see how it is when I fly off to NRT, SIN, BWN, PEN, TPE in the next three weeks.