@DBL2 

It could work together and it has been mentioned here:

Can different Deco models work together?

  @DBL2 

Thanks for the update.

I checked some information and found the previous answer might not be 100% correct.

When main Deco is powered off but the satellite Deco is still on, due to the the whole mesh protocol, the new satellite X55 will stand out and work as the new DHCP server. So the wireless clients connected would still get IP addresses.

But due to the WAN/LAN auto-sensing, since now satellite Deco is also unable to be assigned an IP address from modem either, both Ethernet ports will be used an WAN ports and the wired clients connected to the satellite Deco will be unstable to get IP addresses, which might explain your previous issue"What I am finding is if main device is turned off or disconnected I am not able to connect to the devices cabled into the x55 and the Wi-Fi network drops out."

And as for setting up Deco X73-DSL and Deco X55 separately as the DHCP server, it will come across a similar WAN/LAN auto-sensing on Deco X55 when Deco X73-DSL is powered off. Currently, the workaround i could think of ,would be to keep main Deco powered on all the time or restored power as soon as possible so satellite X55 would no longer start working as a DHCP server. Both Ethernet and wireless clients from X55 will still be able to get IP from main X73-DSL and accessible to each other.

I am also checking with the engineers now and so far hasn't got an update, will come back here if they had any other thoughts.

Thank you very much.