If anyone is looking for a VPN that can be used with all dreamcast online games, I've gotten AzireVPN (https://www.azirevpn.com/) to work. They have openvpn servers that will give you a public IP with all ports open. It's pay as you go too, so no subscription necessary.
I used a $20 router (https://www.amazon.com/GL-iNET-GL-MT300 ... 073TSK26W/) to act as the VPN client and stuck it between my dreampi and my normal router. I had to manually forward ports in the advanced setup (basic GUI didn't apply forwards to VPN connection), but was able to make it work.
Just thought I'd throw it out there for anyone who can't port forward or wants to run two dreampis on the same network.