1) Android only allows you to look for updates once every 24 hours. It has nothing to do with the phone or the carrier.
2) The update comes from AT&T - and only if you're on an AT&T account. Since you're in Mexico, you're not going to get any AT&T towers, and you don't have an account with them, so you're not going to get updates over the air. Go to
AT&T Samsung Galaxy S 5 - XDA Forums and see if anyone has posted an update file. (Update files normally delete themselves after updating the phone, so someone has to capture one.) If you find one, there will be instructions on how to use it in the same thread.