cant connect through VPN with WRT54GS

hi all,
i have this structure modem->WRT54GS->PC
i have this problem,i cant connect to VPN(L2TP) through WRT54GS(error 678), but it work find without the WRT54GS. is there anything i need to set on the WRT54GS?BTW i’m using the WRT54GS V1. hope to get the answer soon.
THANKS