Yes, turning off DHCP Server indicates the device is in modem mode.
Yes, turning off DHCP Server indicates the device is in modem mode.
I switched to a new ISP and it didn’t feel right. My media monkey server stopped working, so I adjusted the DHCP settings on the att modew/router combo unit. The Deco brand mesh connected to everything, functioning as my router before the simple cable connection. Now I’m using fiber at the wall → att box (modem and router with DHCP off) → Deco mesh → unmanaged switch (port 3) → desktop and FreeNAS server. Previously, the att router had a 24-hour DHCP timeout; after that change, it should have resumed normal operation or even started managing addresses again. This likely explains why the media monkey didn’t work before.
It's unusual to notice two distinct IP addresses for the same device. Double-check your network settings and ensure the correct interface is selected. Verify the cascading router configuration matches the intended setup. If issues persist, consult the device documentation or support team.
When you link two routers together, you get double NAT. That’s likely why the server faced problems. AT&T doesn’t allow bridge mode on its gateways—you should switch to IP pass-through mode instead. I’m not sure how to do it myself, but I know it’s the right approach if you’re using your own router.
Firewall > IP Passthrough Allocation Mode > IP Passthrough Passthrough Mode > DHCPS Choose your Decoc router MAC from the list Save
Technically IP pass remains in bridge mode, but you need to set it up manually instead of relying on an automatic option.
The issue with AT&T's setup isn't a straightforward bridge or direct pass-through. It's more of a workaround because you can still adjust the firewall settings, and traffic filtering remains in place. It functions somewhat like a transparent firewall, but only if configured properly—it doesn’t fully allow public IP through. The approach is similar but not exactly what I was expecting.
It seems AT&T tends to be very strict about control, especially when they altered my LAN subnet after turning on IPv6—probably to ensure everyone used the same settings. Making sense for fixing issues, yet they eventually had to turn off IPv6 because their V6 network didn’t work well.