I'd be careful w. homebrew ROMs, keep in mind that there are TWO versions of the “epix style” PDa-phone from SAMSUNG. The Samsung SGH-i907 (the actual Epix) and the Samsung SGH-i780 (the precedessor, released in JAN 2008). Whats the difference ? the I907 has more updated ROMs, more battery capacity and its missing the second QVGA front-camera for webcamming/video-conferencing. (funny though the exec/driver that controls the front cam still exist in the WinMobile ROM, it's called MIRAGEFRONTCAM.EXE but if you execute it, it runs the actual main-camera on the back, just without the camera-GUI).
So, since you've to deal with THREE ROM within the Epix (ROM for Phone-HW, Kernel/driver ROM for non-phone HW like Bluetooth, WI-FI, USB, SDHC/SDIO, CPU and Windows-OS/User-apps ROM) it's not guaranteed it'll work.
As for your WAP.CINGULAR error – i assume it's part of the phone-ROM API to the WindowsMobile. Did you try to turn on (checkmark it) the “Do Not Use Proxy Server” in SETTINGS – CONNECTIONS – PROXYMANANGER ?
EDIT: I know out of experience that the PocketIE by default is accessing AT&T's MEDIANET intranet, which REQUIRES to “un-check”/utilize the ProxyManager, that way AT&T can track you as customer for purchases like games or ring-tones or making chances to your account.
I stumbled over that, since i was never able to check my remaining SMS/minutes of the ProxyManager is turned off (check-marked), using the PocketIE