Hi
I’d be happy to help with your access to BestBuy.com via your cellular data! As posted, it does seem that the site has an issue with its SSL certificate. This is not an issue with your data connection specifically and is not localized to AT&T service. The issue will lie with the BestBuy site itself and access from certain networks. Should you attempt searching the error code that was produced, you may find other examples where users have encountered the same issue, not only on their mobile devices, but in some cases also via their computer.
Please let us know if you are also encountering issues with other websites, preventing access or if you have any concerns regarding your cellular data service.
I hope this helps!
Charise