I never tried this config with the N900. However I do have experience in those kinds of setup (IPsec, SSL VPN...).
Given the fact that when using your "standard" dhcp, it works, I wonder if you have correctly defined the default gateway in your dhcp for openVPN.

The second point of attention is the network range you used for the openvpn. You need to ensure that the rest of your infrastructure konws how to route back your traffic to the openVPN subnet.