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Stick-on boosters are snake oil, don't waste your time or money.
Take off the battery cover. See the round gold connector top right? Samsung claims that's an external antenna plug "mcx" type connector. AT&T says nothing.
But an external antenna won't help you unless you've got a direct view of a tower and you keep your phone at that spot.
Other option is a booster, about $350. You put an external antenna where it can see a tower, cable it to the booster, put the booster in your room or near it, and it works like a mini-cell tower.
Last option--and possibly the one to work for you--is to buy a femtocell or picocell from AT&T. About $100 and you plug it into an internet connection or a phone line (ask them which). That's another mini-cell tower in a box, it gives you your own tower, right in the room. Range should be good for at least 100' in all directions on your floor in an office building.
Cell phones are radios. Radios don't work well underground, underwater, under dense construction. There are limited ways to bring a signal into a bunker.