@DanielW 

You probably have something wired to the main Deco other than the modem, maybe a switch to the other port?

If you do, disconnect that and connect to the main Deco via Wi-FI.

@Tony I have my Motorola modem connected directly to the main deco. I also have a netgear 4-port switch connected to that same main deco. Everything else is wifi or ethernet to one of the other deco units.

@DanielW 

What happens is when the internet is lost the Deco no longer knows where the internet is coming from so the LAN ports get confused. If possible connect the switch to one of the satellite nodes instead.

@Tony I'm not sure what you mean by "connect the switch to one of the satellite nodes instead". The diagram below shows how I have things setup. I would expect to be able to access the the modem interface from my Desktop regardless of whether the Modem had internet access. Instead what happens is that all local network traffic (e.g. 192.168...) stops routing.

   +---------+   E   +--------------------+   E   +-------------------+  E  +-------------+
   |  Modem  +------>+ Deco M9 Plus (main)+------>+ Netgear 1G switch +---->+Phone device |
   +---------+       +--------------------+       +-------------------+     +-------------+
                             | W
                             v
  E=Ethernet         +-------------------------+
  W=Wifi             | Deco M9 Plus (secondary)|
                     +-------------------------+
                          | E
                          v
                     +----------+
                     | Desktop  |
                     +----------+

@Tony @DanielW I have a similar setup, so I'd like to request this as a future feature: 

The ability to set one of the ports on the "main" Deco as WAN only, so if internet goes down, the LAN stays up (even this switches plugged into the main Deco).

It's a "standard" feature of most regular wireless routers because they have a stand-alone WAN port.

@DanielW 

If you were to disconnect your switch from the main Deco, then disconnect the internet, does your desktop still have a local IP address? If so, what is it? And are you able to ping other devices?

@DanielW 

Sorry for hijcaking old thread.

I have the exact same problem. I had internet connection down for few hours and could not connect to my server which is connected by wire to same Deco M9 plus as my modem. It really should have dedicated WAN port if the Deco device gets confused when WAN is down. My other Deco devices are in other locations and can't move the server.

Have there been any other solution to the problem? This issue is very important to me.

@DanielW I never found a solution. If I want to check the modem, I am forced to carry a computer to it and connect it by ethernet. This feels like a high priority bug to me, but it may be an edge case for this type of device...