There's two different unlocked phones. Just be sure you get the one with all the AT&T bands. One of the unlocked phones, they assume you're on T Mobile and it does not have Band 30.
I bought one from Apple before the "SIM Free" version was available, so one of my iPhones does not have Band 30.
If you want to be sure you have all of the AT&T bands, and you also want to be unlocked, then you can buy the AT&T version from Apple and it will be unlocked. It will *not* be unlocked if you buy it from AT&T, however.
What I'm waiting to see is if I ever end up in a place where I have the same visual array strength, say 3 balls of LTE, and I'm getting 25 Mbps on one device and only 1.5 Mbps on the other device.
That's going to be a sign that both iPhones are on the same array, but one of them is on Band 30 while the other one is on the overcrowded band that caused them to roll out Band 30.