Peplink Transit Duo Pro - Bridging LAN and WLAN

Hello everyone!
I’ve been trying to replace a MikroTik Routerboard RBM11G that is acting as a bridge between a FortiAP and a Control System on a vehicle. The picture below shows the situation.


The problem i ran into is that the MikroTik is bridging the WLAN and LAN interfaces and is using MAC translation in Station Pseudobridge mode. So basically it is just acting as an AP that connects to the FortiAP and then via Ethernet connect the Control System.
It feels like i’ve tried everything on the Peplink to mimic this as much as possible, but i can’t figure out how to use the Transit as an AP like this.
One of the problems is that the Transit gets confused when 10.10.10.0/24 is connected on both WAN and LAN.
I can get as far as to get these ping results:
FortiWLC ↔ Transit = PING works.
Transit ↔ Control System = PING works.
Control System ↔ Anything outside of Transit = PING does not work.

I’ve tried:
*IP Forwarding Mode
*Drop in mode
*Static ARP

Something also worth mentioning is that the Control System only has a Static IP of 10.10.10.18, SNM of 255.255.255.0, but NO default gateway.
Does anyone have any experience with a similar situation?

Best regards,
Mattias