First, get the phone unlocked. (See
AT&T Wireless Consumer Device Unlock Request.)
Then, once you get to Portugal, compare SIM prices with the amount of data you get for the money, buy the best deal you can find, put that SIM card into your phone (if you don't need the phone between your home and the airport, leave the AT&T SIM card home - they won't charge you for a replacement if you lose it, but it's a hassle to go to the store and wait for someone.)
Most local cards are cheaper than a US carrier's overseas plan.
See
Portugal | Prepaid Data SIM Card Wiki | FANDOM powered by Wikia MEO seems to be about the cheapest (definitely cheaper than AT&T). If your phone is capable of Bands 18 and 19 and Band 38, you should have no problem with 4G. (3G is 2100MHz.) Just compare what you need with the facts on the link, then go into a carrier store (there's a store locator link for each carrier) and buy a SIM. (You'll have to provide name and address for a prepaid SIM - and a lot more for an account, which you won't need for a temporary visit.)