Sonic.net

Call Sonic and tell them to inform AT&T you want another RG. Specify the BGW-210.

Has anyone successfully done this? AT&T customers themselves cannot guarantee delivery of a BGW-210. AT&T Call support indicates that yes, they can ship a replacement router, and it will definitely be a BGW-210. Then another Pace shows up in the mail.
It'd be great if Sonic could somehow backdoor things such that a BGW-210 would be delivered by AT&T, but I've seen no evidence this is the case.
Requesting a truck roll from AT&T seems to be the only way to get it done.

For what it's worth, you can confirm MTU diagnosis easily by simply dropping the MTU on both ends for your connection. Not a long term solution, but may confirm your suspicions (which sound spot on). MTU problems are particularly rough because it is usually not one device's fault but a combination of multiple devices' configuration :-/

I was struggling with this exact DMZplus bug and finally gave up trying to troubleshoot or convince Sonic and AT&T - neither would take responsibility since the service was provided by AT&T but I was a Sonic customer. Total BS.

I finally just ordered the BGW210 from Amazon, plugged it in and the problem was solved. It worked immediately with no issues or special configuration or activation.

The salt on this wound is that I'm forced to continue renting the AT&T issued modem even though it's not being used and doesn't even work!

Disappointing service from Sonic and AT&T, but I was happy to have solved the problem.