Preamble
I eventually got around to sorting out my IPSec/L2TP VPN, but found it was pain!
Firstly got both Mac and S9 connecting but was unable to see the actual network, both required slightly different fixes.
General
When you connect to your network the ip handed out is on a different LAN. Say normally your NAS is 198.168.1.50, when you connect you will receive say ip 10.7.0.1 from the VPN server on the NAS. The NAS will now have a local ip address on the same network at 10.7.0.0, hence you can still have access to the NAS.
My problem was that I could not see the rest of the network.
For the MAC
I struggled thinking it was a server side problem until I did a traceroute on news.bbc.co.uk, and noticed the route didn’t involve my NAS. There is a little button on the VPN connection setting on a MAC under the advanced tab, under options, that says “Send all traffic over VPN connection”. Clicking this box allowed me to view my whole network, and also get bbc news routed that way also!
For S9
I had a similar problem on S9, but under the VPN settings, advanced, I found I could specify my DNS. And after that I had access to my local network again.
Conclusion
VPN stuff should be trivial, but appears solutions are buried in the details.