I do this all the time....

Using ONLY one network card in the mac - connect the mac to the 'local' side of any 'router' and the internet to the 'wan/internet' side of the router. Artnet as default uses address range 2.x.x.x which IS routable (ie passes though the router - so you must block 2.x.x.x on your router..... or better - use address range 10.x.x.x for artnet - which is either a jumper option. or software (read manual!), which is NOT routable, so no configuration needed on the router.... WIth this method - the routher ascts as the filter between artnet and the internet.

If you need to access the mac, externally through the router, then open a port, use port forwarding or use a DMZ for the mac IP address...

This would be a more reliable method than 2 network cards in the mac.

Simon