You mean you are connecting to a RV120 or you are connecting a RV120 to something else via VPN?
If you mean that you are a client connecting to the RV120 remotely and wish to use the DNS servers behind the RV, just add your DNS servers to NIC properties. I would add you local DNS server IP first then a couple of public DNS servers like 208.67.222.222 and / or 8.8.8.8.
Now everytime you connect you will be able to use the DNS server behind the RV and when you are not connected you would use the public DNS servers.