It is not possible to set up box-to-box VPN between the AC1750 and the FVS338.
If you are within the LAN of the FVS338 and you want to connect via client-to-box VPN to access AC1750 LAN, then this is possible. Kindly read pages 148-157 of the AC1750 user manual here about VPN.
If you are within the LAN of the AC1750 and you want to connect via client-to-box VPN to access FVS388 LAN, then this is also possible. You will need to configure the FVS338 for client-to-box VPN using the VPN Wizard. Then, you will need to install and configure the ProSAFE VPN Client Professional on the PC that is within the LAN of the AC1750. You can download the latest version which is 6.40.00.4 here. As reference in setting up client-to-box VPN, kindly access and read the article below:
Be reminded that the ProSAFE VPN Client Professional has a 90-day trial period. After the trial period expires, you will need to purchase a VPN license key from any Authorized NETGEAR Reseller listed here.
It is important that the LAN subnet of the AC1750 is different from the LAN subnet of the FVS338. For example: if the LAN subnet of the AC1750 is 192.168.1.0, the LAN subnet of the FVS338 should be 10.10.10.0.
