im going to jeju for 4 days and splitting my accomodation 2 days at seogwipo and last 2 days near jeju city. and my friend is going pretty excited for hiking mt hallasan also but not sure how much it costs for how long, where the entrance etc etc.

im not kaki hiking, my friends are. 

Hey, I am going to Jeju in winter for 3 days, should I stay at the city or Seogwipo? I want it to be close to the attractions? Which is a easier base to reach all the popular attractions? How's Udo Island, is it worth my time to visit there, despite having only 3 days. I am prolly spending a day in Mount Hallasan, love hiking hehe. How much are the food over there? How do I keep my spending on food low? Thanks

no issue.
this is the one https://hello1010.my/south-korea-unicom-sim

exactly the same as the one on Klook. You can double check with them.

My experience was good, it was fast, no downtime. Setting up was quick, did it while in the immigration queue.

I mostly used Kakao maps. FYI, Google maps is not accurate in Korea. Look into the Naver and Kakao map/transportation apps and see which one you like more.

I was going to pre-order SIM card on klook.com.my
It seemed expensive - RM236 for 30 days.

Then found https://www.krsim.net/ ~RM190 for 30 days, with unlimited 4G + 100 min voice calls.. AND a local Korean number in case you need to be reached

SIM no need to carry around router. If more than 9 hrs, need to carry extra power bank for router. No need to charge every single night. Also SIM card, people (drivers, restaurants) can call me back

YES. By mid December, the average temp would probably be in the negative even in the day time.

Last year's winter was one of the coldest in Korea's history with temp as low as -15 in Seoul and even colder in some parts. Forecast this year is saying that its going to be even colder than last year!

Even for people that used to winter weather, Korea's winter is bone numbing cold. Best to layer a lot and get the thickest jacket you can. Indoor heating is very good and warm though, so you can adjust your layers accordingly after a few days.

Actually I've been to Seoul and Busan many times d.... Jeju also been there once for spring but it was only 3 days. I totally fell in love with Jeju jor...that's why I wanna go back and have a relaxing road trip. So far I'm thinking to go on my itinerary like this :

Day 1  One night in Jeju-si
Day 2  One night in Hyeopjae beach
Day 3  Seogwipo
Day 4  Seogwipo
Day 5  Seogwipo
Day 6  One night in Seongsan Ilchul Peak
Day 7  Jeju-si
Day 8  Jeju-si