Why I can't find Network Mode Section option in my Samsung Galaxy S-5 AT&T?

I have purchased Samsung S5 at&t mobile, i want to select network mode from LTE to CDMA or GSM, but i could not find this option, any one plz tell me

I have purchased Samsung S5 at&t mobile, i want to select network mode from LTE to CDMA or GSM, but i could not find this option, any one plz tell me what should i do now?? i am in a problem,

or:I have purchased Samsung S5 at&t mobile, i want to select network mode from LTE to CDMA or GSM, but i could not find this option, any one plz tell me what should i do now?? i am in a problem,

or:It's in another section but I can't remember because I was only setting it up for somebody

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