Not sure what you trying to achieve here.
I do not believe these SMB RV series have ability to mentioned DMZ as a VPN server interface as per the document (but happy to listen to someone overrides this and tested).
what advantage you get WAN over DMZ, Yout WAN interface is exposed to the Public and you redirecting to DMZ.
Since this is your VPN client as trusted users, they validated with correct user rights before they get into network.
here is some reference guide for DMZ :
https://www.cisco.com/c/en/us/support/docs/smb/routers/cisco-rv-series-small-business-routers/Configuring_DMZ_on_the_RV34x_Series_Router.html
I am sorry to disapoint you - but i have told what i have learned - Please explain more of your requirement is different.