Different gateway for different interfaces

Dear all,

I'm working on openwrt 19.07.

I have LAN and DMZ working.

I have a subscription for a VPN. my goal is LAN to use the vpn and the DMZ use the WAN (otherwise the local webserver can't be reached from outside)

At the moment I have the VPN connected but all traffic is going out via VPN despite only the lan is set for forward via VPN and DMZ forward via WAN

I have no clue how to "split" the traffic between VPN <-> LAN, WAN <-> DMZ.

Can someone give me any hint?