Every Seoul subway station and most Line 2-9 trains have free 'Seoul WiFi' that actually works for maps, Papago translation, KakaoTalk messaging throughout underground network.

Works in tunnels between stations too, not just platforms. Covers navigation needs from Hongdae to Gangnam without expensive airport SIM cards. Saves 30,000-40,000 won if you're staying central Seoul areas with good subway access.