Port forwarding on MacOS X Catalina for both wifi and Ethernet?

Hi,

I’m trying to set up port forwarding for a particular application on my iMac, and have a VPN that allows port forwarding. The VPN’s instructions for port forwarding aren’t specific enough, so I haven’t been able to get it set up yet.

I want to leave wifi on (mainly for airdrop, and for backup if Ethernet drops out), and use Ethernet for other functions. Can I set up different static IP addresses for Ethernet and wifi? Do I set a static IP by logging in to my router-modem, or in the iMac’s system preferences? Or both (in which case, which first)? Then where do I go to set the port forwarding after that? (Sorry for my woeful understanding of networking!)

thanks for any help you can give.