Trouble in getting Auto IP from ISP

[JWong28]

I brought a N56U two weeks ago. It came with ASUS FW 1.0.18? and worked fine with my ISP (3 broadband in HK). Since I need a VPN at home to be able to access my servers outside, I tried the latest ASUS 3.0.0.4.260 and the Padavan 3.0.3.0-026 (both have VPN and other improvements), but both versions CANNOT obtain automatic IP from ISP. My previous wireless routers including a Linksys and buffalo WHR-G300N can do DHCP with no issues. I suspect this may relate to the Linux DHCP client (both the latest ASUS and Padavan is based on Linux 3.50 ). Any one has similar problem with those FW using DHCP mode and has solution to resolve? All help are much appreciated!