Airport KT booths charge 40,000 won for monthly unlimited that throttles after 1gb daily. Same service costs 15,000 won at Seoul Station convenience stores once you arrive via AREX.

Buy prepaid SIM at any Myeongdong 7-Eleven or CU after arrival. While Line 2 and 4 have decent wifi coverage, having data helps navigating Bukchon Hanok Village narrow alleys where connectivity disappears. Or just use KakaoTalk over subway wifi like locals do.