pfsense, a very powerful open source router OS that supports multi-WAN. So you want internet access on an OPT interface for a second LAN or even a Wi-Fi network, or maybe even a DMZ network?
Assign a static IP address on a separate subnet, other than your LAN subnet, and save.
Go to Firewall –> NAT –> Outbound –> and enable AON. Add an entry for your OPT1 like below:
And add the appropriate firewall rules such as mirroring the rules from the LAN to your OPT1 interface and replacing the network from your LAN to your new OPT1 subnet.