It seems to be hit and miss. There was a time our home wifi was interrupted for an extended time. My wife works from home and telecommutes so having it down while AT&T fumbled around and sent out multiple routers, and then semi-incompetent technicians was really not an option. I do have the unlimited plan and my son downloaded and set up my phone as a hotspot. This worked fine (actually, at times BETTER that the wireless) while they were trying and failing to find the issue with our wifi. Turns out some incompetent person installing another service at the distrubution box somehow disconnected ours. Almost 2 weeks.
Now, if I try to use the same app to activate hotspot, I get the same message everyone else gets, about calling 611 or going to att.com/mywireless. This was after one of their infamous software updates. If they had ever got around to trying to charge me 'extra' for using it as hotspot while they were not meeting their contractual oblications with our wifi, I would have have my legal team involved the very next day.
After being a very early adopter of the AT&T Cellular service, after decades of use and very few issues (other than they always seem to be more than the others), this is my one dissatisfaction point. We bought the absolute top line plan availalbe, and now they don't want to support it. Once they 'sunset' unlimited, as someone said, we will move on to another carrier. Customer Loyalty has no rewards in this arena these days....