My server is invisible to computers on nearby LAN

[WichidNixin 1]

I couldn't think a great way to word the title for this issue as it seems to be very specific and technical.

I am hosting a DayZ server at my cousin's house in a VM. I host it there because he has a fast fiber internet connection.
The VM that runs the DayZ server application is connected to a virtual switch along with a bunch of other VMs that I run on my server.
VMs connected to that virtual switch are assigned IP addresses and get a default gateway from a DHCP server.
This virtual network is separated from the physical LAN at my cousins house by a pfSense Router VM.
The pfSense Router VM has it's WAN interface connected to the physical LAN at my cousin's house. The LAN interface of the pfSense VM is connected to the virtual switch. The WAN interface is NATed to the LAN interface and port forwarding is configured in pfSense.
The ISP Router at my cousin's house has DMZ host enabled for the WAN IP of the pfSense VM so that all unsolicited inbound traffic is sent to pfSense and the the port forwarding rules forward the traffic to the appropriate VM.

This all works great. I can see my DayZ server in the DayZ Launcher, I can direct connect to it, it even shows up on BattleMetrics.
The problem is that my cousin who is located on the physical LAN there can NOT connect to it. It doesn't appear in the DayZ Launcher, he can't join me through Steam, and he can not Direct Connect to the server (when Direct Connecting, he has tried connecting to his internet WAN IP as well as the WAN IP of the pfSense).

I tried running Wireshark on his PC when attempting to Direct Connect to the server and when he attempts to connect I do not see any packets being sent to the IP that is entered in the Direct Connect window. If he tries to Direct Connect to ANY other server on the internet, it works fine, and other internet-based servers show up in the launcher without a problem.

My question is, why cant he connect to the server? Why doesn't his computer even send any packets to the server when he attempts to connect? Does the DMZ host configuration on the ISP router cause some kind of conflict in establishing a connection?

Any help would be appreciated. I work in IT for a living and spend all day troubleshooting complex network and application issues so I am quite adept in figuring this kind of stuff out but I suspect that "Direct Connect" does not simply send packets directly to the server IP you enter but rather sends packets to some other server to make a "request" for the server IP you entered and the DMZ configuration causes an issue maybe involving NAT hairpinning or something like that.