I was there this May and saw no Poker whatsoever. I didn’t do an exhaustive search but I don’t think there is any poker at all anywhere in Jeju except when there’s a big tournament series.
I was there at the WPT last year. The island is nice and interesting but the resort is kinda in the middle of nowhere and very artificial. It's mainly for families with children. I was at landing hotel which was basic, prolly best to stay at the Marriott, but I found all hotels overpriced, as well as everything else at the resort. But prolly depends on the time with the hotels.
But at the wpt the games were good, but the cash game rake was pretty brutal, something like 10% capped at usd 25 IIRC, might be slightly off, but in that area. But lots of gambly bad players. No idea how and how much poker is running at regular times, might not be much. The atms also took like 10 us at not great limits iirc.
Be prepared it's Korea, outside the main tourist spots, most peeps will not speak or understand any English. I can give the insta of a great English speaking tour guide, if interested PM