Which Korea eSIM actually works when you leave the city? I spent two weeks pulling coverage data from Opensignal, official carrier maps, and hundreds of Reddit traveler reports. The honest answer surprised me. In Seoul, almost every eSIM performs identically. But once you head to Jeju's rural coast, Seoraksan's trails, or Busan's outer districts, the network your eSIM piggybacks on matters a lot. Quick Reality Check: What works: Every major eSIM (Airalo, Holafly, Ubigi, KRSim, Nomad) connects reliably across Seoul, Busan, and popular Jeju spots. What doesn't: LG U+ can weaken in mountainous zones, and "any eSIM works" advice falls apart at Seoraksan's deep valleys. Surprise finding: Per Opensignal's December 2025 report, LG U+ actually leads 5G Availability at 90.3%, but SKT still wins nationwide 5G speeds around 1,064 Mbps.