Hi Joe, I'm hesitant to even respond as there are so many variables in what you are asking. There is not only how that 3801 can be configured but also what AT&T will support or what you are willing to set up even if AT&T doesn't officially support it.
AT&T is not in our area so I can't say what's best but you probably need to spend some time determining if the following are possible and supported (if that's important to you).
? does AT&T use PPPoE Authentication? If so, your 655 would have to take over that task if the 3801 is put in a bridged mode.
? does the 3801 support a form of 'Bridged mode'. (I half quoted that as the term is loosely used here.)
? does the 3801 have an Advanced DMZ, Advanced plus or whatever it might call it's DMZ mode.
- in a jam, could you set up the 3801 to not dish out wireless or DHCP and assign a new static IP to it's LAN port in a different private subnet (say 10.10.10.1). Assign a static IP on the WAN port of the 655 in the same new subnet (10.10.10.2), make each opposite device the gateway for the other. This way they do basic routing. Then the 655 could continue doing DHCP for the rest of your LAN in the 192.168.2.n range.
I also agree with Arthur, you need to figure out your proper subnets.
Sorry, that all may have been confusing as I don't know your comfort level in this. If so, try and research on web boards the answers to the first three questions, then we can maybe berak it down into something that makes more sense.
Googling things like "bridged mode on HGV 3801" should bring you to lots of discussion about your exact scenario. It will likely be confusing and filled with contradictory stories but you will likely found out what options like bridge mode or DMZ Plus it supports.
Or if you are truly a patient man, you could call a support line and ask them.
With great power comes Awesome irresponsibility.