It may just need the APN settings set. If you can’t find it via Google, you’re carrier can probably help you. If not, call 718-472-3800 for assistance.
I just activated the S7 that I purchased in March 2017, to the AT&T $45/$40 per month, 6GB plan. The mobile data did not work after activating, but the voice did. I went to this site and set the APN with the 4G settings and now I have data. http://www.thecellguide.com/apnsettingsallcarriersus.php