@MattL 

The easiest thing would be to switch the P9 to Access Point mode so everything will be on the 192.168.1.x subnet. 

If you want to keep the P9 in router mode, you could try port forwarding (in the Deco app) the ports the NAS uses, and reference that. So if the NAS uses 8080 for the HTTP port and has an IP of 1.150 from the router put in the browser: 192.168.1.150:8080

@Tony 

Hi Tony,

Thanks for your reply.  I should have put in my original post that I've ruled out the access mode fix as I want the other features such as parental controls that router mode offers me.  I've had a play with port forwarding but I can't get any joy.  The NAS to connect to its interface uses port 5001 as a default, but I can't hit that, and no devices on the network can see NAS drive for file shares and/or DLNA shares to stream from.

I must admit, I'm at a loss.  Either there's a limitation with the P9 system, or I'm missing a key piece of information in my setup/config.

@MattL 

Have you tested the P9 in AP Mode? If that still does not work then there is another variable.

Your Synology uses way more ports than the one you specified: https://www.synology.com/en-global/knowledgebase/DSM/tutorial/Network/What_network_ports_are_used_by_Synology_services

I would contact the manufacturer and tell them how you plan to use the unit so they can tell you what ports need to be forwarded.

@MattL as I understand it the first configured P9 is the router and every other P9 is an AP.  The ethernet port on an AP should only "see" the local 192.168.68.x network.  I had a Roku wired into a P9 AP without problems.