After hours of trying every possible combination on my Westell 2200 modem from DSLextreme to configure for transparent bridge mode, the solution is to make sure you disable the "Private LAN" DHCP server under Configuration, Private LAN Configuration, Private LAN DHCP server, UNCHECK.
The magic combination is under VC Configuration, VC1 (VPI 0, VCI 35) set protocol to Bridge, and mode to Bridge under Bridge Settings, then disable Private LAN DHCP. This is specific to DSLextreme and will not work with providers that use PPPOE at the modem or router.
Most modems will disable any DHCP server when configured for Bridged mode, but the Westell 2200 does not, requiring you to manual disable it. The more annoying problem is that DSLextreme tier 1 support will tell you "We ship all of our modems in bridge mode", which is technically true, but this modem is in "Routed Bridge" mode which does not allow your router to get the public IP and handle NAT for you. After a couple of chat sessions with tier 1 support I had to call and get it escalated to tier 2 to finally talk with a tech that understood the problem.