Incheon airport, ways to get wifi in south Korea for a few days

I'm flying to Korea in a couple of days, I live in the UK and never been outside Europe, also I'm have never traveled much, been to Italy and UK mostly. I'd like to be able to use Whatsapp, mainly to communicate with my family, and internet access in general, maybe few things like google maps etc.

I've been suggested to buy a SIM card, or even a cheap smartphone to use as router, with a Korean SIM card.

Are these two the only options? Is there a specific type of SIM I should look for? I've read of unlimited plans that can be used for a few days with no need to return the sim, if it is only data (which I assume translates into internet access).

How about buying a cheap phone instead? Is there a particular benefit over just buying the sim card?

(I'm staying for 7 days, including the flight which narrows the 7 days essentially to 5 days).

Also if you've already been at that airport and can suggest a shop where I can look for these that would be great, I don't speak Korean at all, and I'm trying to plan things so I won't get confused.

Thank you