PPPoE over L2TP

[mcit]

I am working on a new project where I need to route a public IP from our office to a remote router, over an LTE connection. This will be used to provide all internet traffic to and from the remote site.

The proposed setup looks like this:

I have sucessfully established an L2TP tunnel from the remote 4G gateway to the PfSense in our HQ, this provides a NAT address, and I can get data to flow across the link.
I need to now provide a public IP to the remote pfsense router. I thought this would be simple enough using PPPoE. The problem I am having is that the PfSense PPPoE server never sees any request for connection come through. The PPPoE client sends the request, but never sees any response.

If there something here I have missed?