Layer-2 firewall that blocks DHCP from the modem seems overly complicated, it's already more complicated as it is now.

Just seems so strange and unnecessarily difficult to me, there MUST be a way around this issue, some quick hack or hotfix or PHP code patch...

They are one in the same address.

101.187 is the beginning of my public IP address.

At this current point in time, doing a major overhaul and change/spinning up a new VM isn't possible. I would just like to see if anyone can tell me what you need to do with pfSense to get it to port forward properly under these conditions.

I have a pretty basic knowledge of networking, this is true. The thing is, the windows server was able to work/port forward in this environment, while the pfSense one is having difficulty doing so. We require DHCP on the pfSense box to be enabled/have it assign addresses as we have DHCP options that need to be set in order for some PCs to discover the PXE server.

Disabling NAT has the effect of preventing PCs on the pfSense LAN from connecting to the internet.

I've left the office and will try Ron's answer tomorrow, just some information, the pfSense DHCP range is 192.168.1.1-254 and the Modem's range is 192.168.15.1, with .2 being the WAN interface of the pfSense box.

Okay, if the secondary NAT is the issue, how do I disable NAT functionality on pfSense, I've just had a peek in likely places, not there.

The solution to me was to add a better router in place to deal with the limitations of the Modem, which had worked until now, which is something I cannot figure out after trying a variety of configurations on pfSense.