Need Modem in Bridge Mode

We have called tech support multiple times and requested the modem be placed in bridge mode and they will not help us. Can someone please help us get this done? We have a Cisco 881 behind it and we need it to pull the public IP address so we can manage it remotely in case the VPN goes down. It is DHCP account but our router is set to DHCP so it should work fine. We have not had issues in the past to get this done. Thanks