I have one of those modems from my ISP that has a router built in: an Actiontec T3200. I connected my Deco M5 successfully to the LAN 1 port and the internet is working fine. However, I have IP Forwarding configured via Deco but my public IP is not reaching the destination server via Deco-- probably because it's using the modem's internal network. This page says "Deco works well with a modem router, however, it’s suggested to configure the modem router in bridge mode to avoid some technical problems, such as double NAT." I'm assuming my issue is the double NAT, and the solution is to somehow get my modem to make the Deco the "main" network.

There is a setting in my modem called Port Bridging, where I can set the Port1 Bridge state to "enabled". When I did that, the internet completely became inaccessible, even after rebooting both modem and Deco. Ethernet cable connected directly to a LAN port on the modem router also didn't work, so I disabled it again. I don't see any other bridging settings available.

Some questions:

1. The Deco is connected to the modem's LAN 1 port. Should it be connected to the WAN port instead?

2. Can I get what I'm after by using its DMZ mode, which says "DMZ hosting enables a LAN device to use the modem's WAN IP address as its own. DMZ places the LAN device outside the firewall."

3. Can I forward every single port on the model to the deco and get what I'm after?

2. What's the best solution here? What am I missing?

For reference, here are the modem's available advanced config settings:

The firewall page also shows that the security level is "NAT only":

All help would be greatly appreciated!