Help Network Mode

I bought an unlocked AT&T Samsung Galaxy S5 smartphone and it is well connected to 4G network (It shows an icon on top), but I don't know if it is connected to 4G LTE. How do I check if I am capable of connecting to this network? Browsing google I found I have to choose it under Network mode, but this option doesn't appears.

Under Mobile networks the following options appear:
-Mobile Data
-Data roaming
-Access Point Names
-Network operators

The "Network mode" option supposedly should appear between "acces point names" and "network operators" but it doesn't.

Thanks in advance,

pd. Im a noob in android phones was a previous apple iphone user.

pd. I am using it in MEXICO