On your 2Wire Modem, you must give the DIR-655 a Static IP Address. In the Firewall Settings, you must set the Firewall to Minimum Security (Low). Also move the DIR into the DMZ Zone of the 2Wire.
This is not correct. The RG would assign the same public, external IP it gets from AT&T to the device in the "DMZPlus" mode. No static IP assignment is necessary.
I have similar, but a little more complicated setting, and it works fine (except some problems I am having lately with the DIR-655 unit itself, why I came visiting the forums in the first place).
RG --- DIR655 --- linux box
| | --- box 1, box 2, ...
| | --- switch --- more boxes
| | --- wireless clients (many)
|
| --- VOIP ATA
| --- TV1 over cat6
| --- TV2 over coax
RG sets DIR655 is in DMZPlus mode. Wireless is off in RG (but must have DHCP to serve the televisions and the VOIP ATA box). In DIR655, DHCP server is off. linux box provides DHCP services to the entire network. The above setup works for me even if I move the DIR655 out of DMZPlus mode (however, bittorrent clients complain a lot. I don't use them much - so it's okay).
That said, I don't really know why you are having upload problems. I must say I regret getting UVerse. I had AT&T's plain old DSL service for many years. Only 1.5mbps DL, even more sucky UL, that was. But it was a dumb, stable pipe. I miss it.
I do not have need for esoteric services like gaming, p2p etc., and can't tell how they might work. But I do massive uploads (from the linux box, over encrypted rsync for remote offsite backup), and it works as advertised.