Sorry I didn't see this sooner. I think sometimes subsequent comments are not always emailed to me to reduce the number of email notifications.
I don't know if the track interface will work behind another firewall unless you can somehow use delegated prefixes from the other router you have on your network. The ISP provided modem/router is not always the most feature rich. They seem to only have the most basic features available for you to manipulate (hence why OPNsense/pfSense is awesome). My guide was written as OPNsense being the main router. When you run OPNsense behind another router, it complicates the set up and you may not have all features available to you or you have to go about configuring them differently because you are now on a network that's behind another network. Having OPNsense as your main top level router enables you do more since it's the main entry point into your network.
I know we have had some discussions on Twitter, but I wanted to reply for others to see.