Two things to do when you arrive.

When you arrive there are two things you should do. No matter how long it takes, don't get in a hurry this is Sri Lanka trust me whatever it is can wait.

One is exchange all your money. Don't wait and try to do it later. You won't get a better rate and banks here are a huge pain in the ass.

Two is sim cards. Go to Dialog. You will have a long menu of choices of data plans so ignore them and ask for 100 gig. And buy a extra sim card just in case they are crazy cheap.