At&t Galaxy s7 with straight talk mobile data issues

[AC Question]

So I bought an at&t Galaxy S7 off ebay and put in my sim card from my old phone which was an at&t moto x gen 1. It worked perfectly for a while after I updated the apn settings, but then the mobile data just stopped working. I can still call and text, but I can't use my data. I never had issues with my old moto x, so I don't know what happened. Any advice would help, thanks!