The hardware in question is a generic, "white box" intel i3 unit with a total of 6 copper ports. No SFP or other networking ports. There is nothing else involved in the equation here. No other DNS servers, nothing. The first copper port from left to right is igb0 and that is what the WAN address (statically assigned) is connected to (ISP modem). The second copper port from the left is igb1 and it is LAN. Again, the OPNSense instance itself has internet access, because I can go to 'firmware' and run an update and it will go out and grab available update packages without issue when I perform an update check.
I really wish the OPNSense team would do more testing and validation.