This was common with the Nexus 4 also. You are probably getting it when your phone is downloading data. Did you do a speed test? You can also view status while downloading something and it should show it then. Setting, About device, Status, Mobile Network type. My Network type on my S4 says LTE:14
Did you input the APN? I have posted it below for you if you need it. Give us some other thoughts on the phone?
Name: ATT Phone
APN: phone
Proxy: Not set
Port : Not set
User name: Not set
Password: Not set
Server: Not set
MMSC:
http://mmsc.mobile.att.net
MMS proxy: proxy.mobile.att.net
MMS port: 80
MCC: 300
MNC: 410
Authentication type: None
APN type: default,mms,supl,hipri
APN protocol: IPv4
APN roaming protocol: IPv4
Enable/disable APN: APN enabled
Bearer: Unspecified