Installed OPNsense on my custom built desktop PC with an Intel NIC with two 2.5Gb/s interfaces.
After mulitple hours of troubleshooting, managed to connect the router like so:
Client Machine –> Wi-Fi Router (in Bridged Mode) –> Opnsense Box –> ONU Modem/Router Combo.
I could connect to the Wi-Fi Router and the ONU Modem/Router Combo, but could not gain Internet acccess, even after setting the correct gateway address.
It was an issue with double NAT. Even after disabliing the NAT settings in Opnsense, I still could not connect out.
I will continue this in the future, once I am with an ISP that only provides a dedicated ONU unit and I can then install the router with success.