from Prepaid Data SIM Card Wiki: Korea has an excellent network of open and commercial ?WiFis.

from changwoo[m]: non-korean data esim providers sell much cheaper plans. If you don't need a local number, I don't recommend korean carriers.

StefanoRivera used an international eSIM for a day, before giving up and buying a local one, because it wouldn't stay connected. This was a ?KeepGo Tucana. YMMV

Local plans

If you want a SIM card (Or eSIM) from Korea local carrier, you can get one from one of the below. All 3 are available from stalls in the airport arrivals hall, and usually come with a free TMoney public transit card.

SK Telecom 20Mbps Unlimited

KT 3GB / 5Mbps

LG U+ 3GB / 5Mbps