We could not ping a Synology NAS over VPN. All subnets worked while on the LAN which left this as a routing issue:

1. Log in to DSM (as an Administrator), then go to Control Panel > Network (under Connectivity) > Static Route.
2. Click Create button.
3. In Network Destination, put the (virtual) subnet of your OpenVPN server - e.g. 10.8.91.0 or whatever your OpenVPN subnet is.
4. In Netmask, put your OpenVPN subnet netmask - e.g. 255.255.255.0 in my case.
5. In Gateway, enter the IP address of your OpenVPN server host - e.g. my router's LAN IP in my case, 192.168.2.1.
6. In Interface, choose the relevant interface - e.g. Bond 1 for me, as I use link aggregation for my DS.
7. Click the Create button in the dialog box, and you should see your new route listed as 'disabled'.
8. Click the Save button and it should change to enabled.

https://community.synology.com/enu/forum/17/post/83527