We have a very similar setup running at several locations with other models of Peplink Balance routers.
To make this work, you need to do some careful pre-planning, we recommend moving everything on the LAN network interfaces over the VLANs (FW 6, 7 & 8 support this).
You will need to assign the SFP port to behave like a LAN (not WAN) if that will be your backhaul, this SFP will need to operate in Trunk Mode. Note: in your image, you have highlighted the Ethernet WAN Ports, not the SFP Ports.
Then you will need to assign one of your Ethernet LANs to your reserved VLAN for WAN traffic at the switch and patch this one to each of the WAN Ports (you can duplicate this to a second LAN & WAN if needed).
Setting up the VLANs is the most important part, once setup, the rest of the configuring of the router is made easier by working out the VLANs first.
I can not recommend highly enough to use InControl2 to build this setup with and manage the VLANs. Even if ultimately the solution is going to be within a closed network with no public internet access.
Happy to Help,