I don't know your specific situation, but you say "right out of the box. If that is the case you can request a non customer unlock from AT&T and in most instances they will unlock it.
You should do this before you even take it out of the box, certainly before ever turning it on.
Even if they don't unlock it, in my opinion it is best to wait a few days for an unlock code rather than use as is. Depending on the device it is much more universal and will retain more value unlocked.
Other than that it should work OTB with a LTE SIM, not so sure it would with the Global SIM?
The FP LTE sims run off att so should run on a locked ATT phone. The gsm providers are not as locked down as sprint. But yes follow what was posted above as usually ATT will unlock unactivated gophones.
They have been unlocking unactivated gophones for some time and it is often discussed on slickdeals - contrary to their policy but they allowed it nevertheless. Not sure if things have changed recently or not but I remember seeing a gophone iphone 5s that people were getting unlocked around Christmas time. No harm in trying. But for an ATT freedompop sim you should be fine in a locked device.
I do not have this device and was just trying to see how exactly the policy works.
I simply went to the website to see what the policy was and saw a reference to eligibility requirements and clicked through several links that brought up a selection of phones. Then I picked one at random and followed the steps to request an unlock. The wizard stopped me at the precheck stage by asking if the phone had been in service for 6 months.
I bought an iPhone 5s from Cricket Wireless and was trying to unlock it from AT&T before activation. The request was rejected 6 days later with an email saying "Thank you for contacting AT&T about unlocking your Apple iPhone 5S Model A1533 device. Because of certain AT&T guidelines, we were not able to unlock your device." I guess that I have to use it for 6 months in order to unlock it following the rule.
GoPhone is not Cricket and Cricket devices have always been tightly locked down. While Cricket devices have failed an unlock attempt GoPhones were passing with hardly any issues. GoPhone policy may indeed have changed, but until you actually go through the process, unlike what oldbooks posted, you will not know for a fact..