You can use the original IB4 with the pfSense in Double Nat mode with a cascaded Net, or you can buy the the Zyxel XGS-PON Bridge with the pfSense Appliance as Access-Router itself.
The idea to buy a hole XGS-PON Router for only putting it to bridge mode is not longer needed since the first XGS-PON Bridges came available on the market.