If I unlock an at&t iPhone 6s in Nuevo Laredo Tamaulipas can i still use it when I go back to Texas a ?

How do you intend to unlock it? If you take it to a 'shop' to have it 'unlocked', they're going to hack it. That forfeits all rights to support for that phone and risks turning it into a brick, not to mention completely compromises the security.

If it's locked to AT&T, only AT&T can unlock it.

How to unlock your iPhone for use with a different carrier - Apple Support

Once it's been legitimately unlocked, it can be used with any compatible carrier.