Hi Benjamin,
Access rules between these zones would be a security risk so your option here is to connect to the DMZ servers from the LAN using the public IP of the firewall.
So, the traffic will go out to the WAN and then from the WAN to the DMZ as if you were connecting from outside.
To achieve this you can create a loopback NAT policy to translate the source IP of the LAN PC's to the WAN interface IP and then translate to the server.
This is the most secure way to communicate.
You might as well want to involved you networks team to provide a better understanding of these environments for communication, if not Google it.