Paying for 50Mbps; getting 17Mbps or less.

Have you always gotten low Speedtest results? Are you testing wired or wirelessly? Check using a wired connection. Try different speed tests and servers within the Speedtest sites. If you get better results from different servers then that one server is poor. If you get low speeds from all Speedtest sites, call att to have your line checked. 

Speedtest.net

Gateway model?

Testing hardwired (Ethernet port of gateway)?

Wireless

589 is 2.4Ghz b/g/n

599, 5268 and 210 are dual band.

If dual band can your testing device support wireless AC, if not your testing on 2.4 which generally tops out in low 20s.

If your device can support 5G, are you connected to the 5G network of the gateway. May have to change the SSID of network name, then connect on the new SSID created.

Use speedtests.net to a local ATT server.

Testing off network can result in different routingeading to increased ping, lower speed. For the Milwaukee area I test to Cicero, Illinois outside Chicago.

Having verified and corrected to above how does your speeds look?

 Your listed as being in Maryland, there is no Uverse 50M internet in the NE. Your closest ATT state is Ohio part of Ameritech region or North Carolina part of Bell South.

Am getting very slow speeds in the late afternoon and evening, even when testing with coaxial cable, similar to an incident in Alaska which turned out to be a neighbor splicing into our cable connection. 

How can you believe we're in Maryland when we're right here in Texas? 

 @MormonYoYoMan

When checking your name profile states your in Maryland ...

If your profile is incorrect, you should correct it.