Trouble whit OPenVPN and routes

I assume the DHCP server is also the gateway of your network and your NethServer has just one green interface. In this case you need to set a static route on your router.

If you can’t set a static route:

I think using OpenVPN custom routes is wrong in this case but to answer your question, you just need to enter the CIDR like 10.0.15.16/32

From the docs:

CustomRoutes : a comma separated listed of CIDR to be pushed as extra routes to VPN clients