It turns out the wifi coverage on the AT&T modem is so bad that it's unusable in our house. So I tried to put the modem in bridge mode and use my own router, only to learn that it doesn't have a bridge mode. So I turned on DMZ in order to deconflict the DHCP and double NAT issues only to discover (after hours of troubleshooting) that DMZ mode on the 5286 modem causes your speeds to be capped at ~ 50 down and 100 up.
This is a KNOWN firmware issue (check the hundreds of posts on the AT&T forums - https://forums.att.com/t5/AT-T-Fiber-Equipment/PACE-11-1-0-531418-DMZ-Issue/td-p/5700776) which is being addressed by AT&T for their own customers by providing a different model modem. However, when I submit a ticket with Sonic for the same issue, I'm told that AT&T won't fix the issue for me because "DMZ is not supported".
TL:DR: I was sold a terrible AT&T service, disguised as Sonic gigibit, which in reality runs at 50 Mbps max download uinless I'm willing to give up wi-fi capabiliy.
To add insult to injury, because it's an AT&T service, I have no way to interact with the actual service provider. When I call AT&T they have no record or me and eventually just hung up. I have a contract with Sonic, but they are incapable of even seeing the modem settings, let alone ordering a new device.
I was super excited about gigabit service, and so this is really disappointing. I'm sure Sonic is doing good things with their own network, but as a "Sonic" AT&T customer I feel a bit duped, with no recourse for viable service/support.
(Edited for vitriol reduction)