RBK963 wont allow Synology NAS to access the internet

I recently upgraded our Netgear Orbi RBK852 system to the Orbi RBK963.  On initial setup of the 963 I upgraded to firmware version V6.3.5.8_3.2.19.  I then proceeded to set up the fibre internet connection and port forwarding rules for the Synology NAS.  My intention was for a seamless transition from the older Orbi model, but it has been far from problem free.

The Synology NAS doesn't seem to have access to the internet (Error on Synology:  Network error occurred. Please check your DNS and network settings), even with all required ports added to the port forwarding rules on the Orbi.  Ive tried setting the NAS IP as the DMZ, still no luck with it accessing the outside world.  Ive tried using UPnP for the port forwarding rules - This populates all necessary ports, but still no change.  Ive tried several factory resets of the Orbi.  Ive also tried mirroring the configuration from the RBK852 and it makes no difference.  Ive tried two different Synology NAS on the new 963, the same issue.  If I plug them in to the 852, the issue goes away. 

On the Synology side ive tried static and dynamic IPs, manually setting the DNS server (1.1.1.1 / 8.8.8.8) to ensure it wasn't a DNS issue, and also did network resets on the NAS units.  As mentioned above, on the Orbi 852 theres no connectivity issue, the moment I plug them into the 963, all external connectivity is gone - I can still access the Synology internally though.

Ive spent well over ten hours now testing every conceivable configuration and im at a loss.  If it wasn't for having another Orbi to test with in the RBK852, I would assume it was a Synology issue, but as two seperate Synology NAS units work fine when plugged in to the old router, its clear its an issue with the new 963.  

Can anyone shed any light on this?  Could it be a firmware issue?  Faulty hardware?  Or have I missed something.