@PeterTheDane 

I have the same problem. Can't even find any info whether this just actually is a bug. 

Every client I look up has "Connected to: Uknown". But in the overview shows up under Main Deco.

I have Deco M9 Plus with three satellites (one wired, the rest wifi), latest firmware.

  @PeterTheDane 

Is the Main deco set as a router or access point mode. So does the 5G router run the DHCP and giving out the IP address or does the M9 plus main deco do that as well ( creates a double nat situation and makes the network more complicated for devices. 

If your Deco m9s are up to date then you may have the option to set a preferred deco to a device. Mine deco m9s allow selecting a preffered deco and band for each device.  I would also turn check the status of the Smart DHCP option under the advanced menu in the app , mine is off and run the Deco in Access point mode as i have a ISP Router. 

Switch off the Deco M5 as this is the odd one out in case its causing some weirdness and make your firmware is all up to date. I do have a few devices listed in the app that show as unknown ip address but still work fine on the network. 

I have 3 deco M9 plus  over 3 floors and have had in the past all the devices connect to the loft decos ( furthest away)  leaving out the main deco on the ground so its does happen.