VISA to transfer from Term 2 to Term 3

Hello,

I would like to know if i need to obtain transit visa to go from Term 2 to Term 3 please. I will be using airpot shuttle, two different airlines, second flight will be to Busan, South Korea, time in between is 12 hours.
Also, would i need to go through immigration in order to use the shuttle please?

thank you,
Yana