@Ranb 

There is something strange going on, because with that network diagram there is no way devices connected to the switch will get IP addresses from Host Router.

This is what I would suggest: configure Deco mesh to run in Access Point mode. That will bring all devices to the host router's range (192.168.1.xx), does not matter how and where they are connected, and they will be able to communicate with each other.

See Switching a Deco to Access Point (AP) Mode

  @Alexandre. 

Hey!

Appreciate the speedy response!

Since the network is stable now, I'd prefer not to lose the Deco's enahanced router functions by switching to the Access Point mode.

I can actually access all devices on both subnets for now, so everything's good.

Many thanks again,

-Ran