Previously there were issues with roaming phones working at all in Korea. Some providers didn’t work because their tech didn’t support service in Korea at the same capacity as you might need for an acceptable data speed (for ex, your plan allows roaming in Korea but is slow and limited to 3G).
Nowadays more phones are being released worldwide on carriers that support korean sims and their UMTS, LTE bands even if it is different than the ones used in their home countries. A couple years ago my T-mobile iphone didn’t work in Korea but now it does.
someone can probably explain this better than I can, but if you have international roaming for south korea available with your provider and your phone is relatively new, it should work and you don’t need a sim/wifi egg alternative.
