I have had ATT Fiber in my residence for 3 years and the entire time it has been very slow. Typical download speeds have been 2-3 Mbits pretty consistently, with sometimes the speeds going to 10-30 Mbits. Currently I have the 300 Mbit package. But this was the same when I had 50 Mbit and 100 Mbit. I am standing right next to the router, on a clear channel and I keep getting these bad results. Sometimes the upload result will be 40 or 50Mbits, despite the slow download. This is still well below rated speed, but it is faster for some reason.
I have called support several times and sometimes they say there is a line problem, but the techs that come out don't seem to detect one. They have replaced my modem more than once and it hasn't solved the problem. Anecdotally it does seem worse on the 2.4Ghz band. But then again, there aren't *that* many networks around me even if there were I should be able to get more than 2-3Mbits download standing next to the router. These same devices get much faster speeds on other more congested networks at my office and school.
Is there something else I can do here to improve things? I have a PACE 5268 modem. I only have an iPhone and iPad connecting now, and a fooBot temperature/airquality monitor. Not a lot of demand on my network.
You might see some improvement by disabling IPv6 on the AT&T router. Also a wired connection will avoid any wifi interference if you're unfortunate enough to be in a noisy wifi web with lots of neighbors hosting their own networks.
Overall however the service will never be as fast as your plan name would suggest since you have actually been allocated a portion of a shared channel with an upper rate matching your plan. The quoted plan speed is a hypothetical ideal if you were the only one using the channel.