T-Mo Simple Choice Plan will get you free global data in SK, but it will be slow (maybe, it will be faster next July as it's over 7 months away ...) but for about $5 USD a day, rent the WiFi egg at the airport (arrival hall, landside - one of the service center is open 24/7) for unlimited 4G/LTE access.

We saw "5G" on our devices but obviously not connected to or using it - but, "ordinary WiFi were ultra fast, as in 75 MB up/down-load and at the Novotel that we stayed at, we got over 100 MB u/l and d/l speed, no that is NOT a misprint or typo, we've getting close to 115 MB u/l & d/l over hard-wired ethernet in our rooms - amazing (and, I setup my own TP-Link travel router w. the firewall ... )

Smartphone access over T-Mo was free but painfully slow, it was perfectly functional. But, for the daily price of a cup of Starbuck in Myeongdong, with a nominal & refundable security deposit - tethering enabled to share (up to 4 or 5 users) - I highly recommend the egg. Also, Skypass members flying on KE, upon presentation of the BP on arrival - sometimes ... get discounts & special promo on WiFi hotspot rental.

On departure/leaving from ICN, just drop that off at the 24/7 airport landside counter - and you are on your way. Best way to do this is to reserve it ahead of time - at least 3 days & everything will be set and ready for pickup on arrival. (That is, unless you will be activated on Project Fi with full data speed by next summer ... got my "Invite" already & weighting the pro's and con's)