help to configure Forti with Modem Cable (bridge mode)

Hi guys I have y my small business Modem Cable connection (Modem model CBN)... I put it in bridge mode so it has public IP. Well, first I check modem cable bridge mode with a PC and It´s OK, the PC has a external IP 85.136.x.x The problem is when I connect modem cable (port 1) in Forti 40c (Wan1)... - Modem cable in router mode with forti Wlan1 in DHCP mode, OK - Modem cable in bridge mode with Wan1 in dhcp mode, don´t find external IP 85.136.x.x so KO - Modem cable in bridge mode with Wan1 in manual mode, put external IP / subnet and don´t change default Forti DNS so KO too Question is: What can I configure Forti to navegate with connect PCs? I think that a problem with forti routing configuration but I don´t know...