Missing Mobile Network

I have no option to go to "Mobile Networks" How do I get to my mobile network area? I need to change my network to 3G instead of CDMA which it shows now. My sim card worked when I first placed it into my phone & it offered AT&T network, but Life Wireless my carrier told me to put it into slot 1 as I had it in slot 2. Now it doesn't recognize sim card at all and any area that gave me Mobile network option is greyed so I can't change anything. Here is my phone info
Platform
CPU:MT6753 64bit Octa core 1.3GHz
GPU:Mali-T720 MP3
OS:Android 6.0
Network:GSM/WCDMA/FDD_LTE(2G/3G/4G)
Frequency Band:
GSM：850/900/1800/1900（band5，band8，band3，band2)
WCDMA: 2100/1900/1700/850(B1/B2/B4/B5)
FDD: B1/B2/B4/B5/B7/B17
New phone please help!....... Got phone 3 wks ago & had to wait another 2 wks to get my sim card from Life Wireless.