Upstream Gateway?

[LukaPix]

How would i configure my OPNsense if i wanted to use another gateway, that is currently in my LAN network, to which OPNsense is connected.
I am in the process of setting up and configuring but i cannot swap over the WAN connection yet.
Still i want to install packages and update the firmware, and want to keep access to the Web Interface.

Current configuration Modem/Router <- UnifiDM/Gateway&Firewall <- Switch <- OPNsense Lan port.

I don't want to set custom routes or anything just want my opensense to use the existing internet connection over the LAN port while still being accesible over the web-interface.