Yes, AT&T is maintaining the 3G/UMTS and 4G/LTE networks while upgrading to 5G . Your 6S will continue to operate on 3G and 4G. The current plan is to take down 3G in 2022 but 4G will still be in place after that.
Read moreIs iPhone 6S a 4G or 5G?
The iPhone 6 and 6 Plus are both 4G capable devices . If the mobile internet connection is 4G rated, the handset will show this next to the signal indicator in the top left-hand corner of the display. 3G, E or GPRS may also be displayed, but these represent a slower connection speed, dependent on location.
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