iCloud Locked AT&T iPhone 6
iCloud lock is properly called Activation Lock and doesn’t have anything to do with any carrier:


Getting rid of icloud on old iphone


The owner of that iCloud can release the device from any web browser they log in to their account.

If the phone is carrier locked - you’ll need to work with your new carrier or the old carrier to resolve that if you want to have cellular data on it.

With that out of the way - you can always use a carrier locked iPhone as an iPod Touch and no data connection. That lock only prevents cellular data and service. You can’t use an Activaytion Locked device since iTunes won’t restore it and Apple won’t sign an download to run the OS on a locked / presumably stolen device.