When I configured the new container, I matched it to the firewall forwarding ports which is definately 20988. 28967 is pointed to a different internal address on the Synology. I re-followed the You Tube video from IX about how to set this up.
I can also confirm that traffic is coming in on 20988.
Rebooting the NAS with net.core.rmem_max=2500000 set as per UDP Receive Buffer Size · lucas-clemente/quic-go Wiki · GitHub and the error message in the logs and the QUIC comes up OK
It just takes a while to reboot as I have to migrate VM’s away from the shared storage before the reboot.
So was it the reboot OR the sysctl entry or a combination of both? Dunno