After 5GB on the ATT "unlimited data", which I am 100% certain to the point of being willing to bet money and/or my life that you have never reached it on mobile data, you are dialed back to dialup speeds. Literally from 12mb/s to 512kb/s. That's what "unlimited" is and why it's in quotes.
You could hit more than 5GB on wifi, but if you hit 5GB through mobile data and don't have tethering you are sent back to 1989.
If you pay for tethering, you don't have the same unlimited, and means you are not on the unlimited data plan, either. You're also paying twice for the same usage, so enjoy.