DDNS and VPN

Hi,
I have another of my strange questions...
My current status is|
Asus AC68 manages DDNS
My NAS has a VPN connection
If the VPN is down, I can reach the NAS and all from WAN
If the VPN is UP, I obviously cannot reach the NAS, only other ports in the network.

Is there a solution to this?

Thanks,

Maurizio