Dynamic DNS client uses ISP WAN IP instead of ExpressVPN IP

[lansmurf]

With succes I created two Dynnamic DNS Clients in pfSense, one with a DDNS from Dynu and the other from NO-IP. I created two to see if there is a differeence…
With every interface I use (WAN, ExpresVPN or group of'VPN's) pfSense uses my ISP WAN IP as "Cached IP" to send to the DDNS provider. Which both succesfully connect to the correpsonding domain name. However I would like my Express-VPN-IP to be used. The purpose is to access my cloudserver from outside.

1)Why does pfSense not take the (public) IP of ExpressVPN?
2)What should be the way to do this?
3)Or should you just creat an own OpenVPNserver and connect to that instead?