@Stumpee 

Deco mesh does not like devices with static IP addresses configured on them.

I'll tell you how I configured my Synology NAS, and it runs just fine since the beginning. You decide if you just follow my steps and have it easy, or not.

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Both ports on my Synology NAS are configured as "Get network configuration automatically (DHCP)" .

I am using only Port 1 (LAN 1). Port 2 (LAN 2) is not connected.

Port 1 is wired directly to the router (Main Deco).

I verified I can ping NAS and access its Web interface using its current IP address (Deco app will report NAS IP address).

Having all above steps completed successfully, I configured reserved IP address for NAS on router: How to configure Address Reservation on a Deco

  @Alexandre. No issues with address reservation.  DHCP works fine.  The Synology remains inaccessible with either static or dynamic IP

  @David-TP --

Thanks for the reply. :)

The current setup is just for debugging purposes. While the Deco does offer DHCP to the NAS, no communication between the devices has worked regardless of configuration after that initial handshake. Changing the NAS ports to static IP was just to remove variables from the equation in case it was some sort of DHCP parameter or configuration causing me grief. This one has me truly puzzled.

  @David-TP -

I had one last thought: I had docker running with Portainer with a MAC Vlan.  I thought maybe Deco objected (even though I can't imagine why).  I have since shut down Portainer to remove that from equation.  Can't really remove Docker, but it is what it is.  There are no other port consuming services running on the device.

At this point, as I have access, I can move the data to another machine or even cloud based storage, but would seriously like to understand why I cannot connect through Deco.  Truly bizarre.