Bridged modem connected to router via VLAN

I would like to virtualize a PfSense router that is now a physical box, without having to pull extra wires. The problem is that there are several walls between the modem and the server room, and neither device can be moved.

I don't fully understand how a modem functions in bridge mode, other than it 'just works' when plugging it into the WAN port of a router. So I just went ahead and made a test setup with wherein the modem interface is directly connected to a static acces switchport in VLAN 7. The PfSense VM has one trunk interface, on which several VLAN interfaces are defined. This VLAN 7 (and several others) works and is accessible from the switch port as well as the virtual machine.

But as I expected, the PfSense VM's WAN interface doesn't get a WAN IP from the modem. I've read somewhere that the modem tries to negotiate with the first MAC it sees, so perhaps the modem is trying to hand out a WAN IP to the interface of the Cisco switch it's connected to?

I hope anybody knows whether or nor this setup is possible.

Here is a quick drawing of the test setup