Q: Unlocked AT&T iphone working outside USA

Different countries use different LTE bands. It should work on any compatible GSM network, but it may not work on LTE networks outside of the US.

http://www.apple.com/iphone/LTE/

Bear in mind if you use an iPhone in a country other than the one it was purchased in, the iPhone warranty will not be valid, and Apple may not service the device should it need service.

(The EU is considered one country for this discussion.)

It will work anywhere in the world. The iPhone 7 is the first iPhone to support all LTE bands, regardless of where you buy it. Note that there are three models, two that support specific CDMA carriers + GSM/LTE and one that is GSM/LTE only. But that doesn't affect GSM or LTE coverage on any phone.

Tim, see my post below (derived from your link)

Thanks. That's a new twist on things. I had heard that there were 2 different chipsets being used, an Intel chipset that would not support CDMA at all, and a Qualcomm chipset that would. Didn't realize that they would both support all LTE frequencies.

it's a first for iPhones, and I think it's a good move. I suspect there wasn't room for all bands with earlier models. As evidenced by the fact that the SE has several different models. So it may be yet another benefit of dumping the headphone jack.