An unexpected side-effect has been that LAN devices won't communicate with anything on the Deco network.  

Internet comes in via the ISPs Cable Wifi Router, wifi has been disabled.  One ether port is running to the Main Deco, another is supplying a business desktop in my office and lasty another desktop which acts as a plex server.  Previously I could connect between Plex Server and business PC and any of the various chromecast/tablets/laptops etc. to swap files but no such luck anymore.

Wasn't high on priority to fix it cause mainly affected wife & kids streaming from Plex.  That was until earlier once I realised that I've lost connectivity between business devices (desktop ===// X //=== plotter, SLA Printer, X-Carve).  I've got about 1 week to resolve it before it affects my customers or my wife discovers Plex Pass.

Bear in mind my router is old (5+yrs / pre-WPS button era) because my ISP is hopeless and won't update it.  Its also a NETGEAR CV series so its basically a featureless piece of crap regardless of age.  Bought the CR700 via grey-market in the hopes that I could convince my ISP to add it but no such luck.

Thanks in advance for any help, I know could just installed two wifi cards in both PCs and it would solve the problem but thats Plan B.