I do DMZ for dreamcast, port forward for Netlink.
If you're dead set on DMZ, you could try setting the VOIP adapter's IP address to 192.168.x.98 (I'm assuming you're using the dreampi?) and just keep one plugged in at a time. Some routers might not like that though.
PSO Dreamcast 100% Legit: Eda FOmarl, Automaton HUcast