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.

Hi Romeroarely67!

When you unlock an iPhone (any model from anywhere), you can use it in other countries other than just where you unlocked it. For example, If I wanted to unlock my iPhone SE in Canada. Unlocking is when you remove the SIM lock that mobile service providers (such as AT&T), meaning that a SIM card that is NOT AT&T can provide your phone with service when it's unlocked! All you need is another SIM card that works with your iPhone (microSIM) from, let's say, Poland. Then you're good to go! To remove a SIM card from your phone, use your SIM remover tool that was included in your iPhone box. Pop out the SIM card tray (you should see a pin hole on the tray), then you can swap the SIM inside for any other SIM card you want to use.

Maybe. Someone on here would know. Are you a resident of Mexico?