Why would your fiber provider have you connect to a VLAN on the WAN side?
I've noticed that too with optical home network ISP, they are defining VLAN tag and then they ship a locked preconfigured and rebranded bridge or router device that talks to the optical network ISP only with VLAN tagged frames.
Optical data links that are in plain normal access mode VLAN i've seen mostly through business packages.
I have another suggestion, something that came to mind, maybe the m0n0 WAN NIC doesn't allow VLAN tagged frames, that might be the problem, check it with packet sniffing system, or just try another NIC.