Went thru this just last month ... ICN arrivals all empty into the main terminal on the same (lower) level thru doors marked A,B,C,D ... rather typical international arrivals layout and very clear. You can't completely predict which hall a flight will use for baggage and immigration (my wife and I both on DL flites from US, one in thru C, one in thru D) so a seating spot just outside whichever one you decide is a safe way to go, they're separated by about 50 yards. There is a Kraze Burger just outside the D door, which was our spot. There's also a BIG Karaoke stage (!) between C and D doors. At that spot an escalator to the upper level -- all open atrium (view of everything and everybody) so up and sit down on the benches there would be good, too.

Wifi thruout the terminal was my experience.