DrSkum88 said:

Actually re-reading your post I think you have a different issue.
Are you both trying to use the same account?

Hey DrSkum88, we're using different accounts...

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Working through this with the AI, I set up port forwarding by configuring my computer and my wife’s computer to port 3074 and redirecting externally to ports 3074 and 4074. I also tried redirecting to the secondary port that Plutonium uses, but without success. Another important point I noticed is that both my public IP and my wife’s are the same, and this was my final conclusion: the Plutonium Launcher doesn’t allow a session with the same public IP address, which explains why, when I use a VPN, my wife and I can connect simultaneously, but when I’m not on the VPN and one of us tries to log in right after one of the computers logs out of a Plutonium session, it takes a while for the server to release the session’s IP address... I managed to play by tethering my phone’s internet connection… But it’s not ideal because the connection fluctuates a lot. If any developers are reading this post, would it be possible to add an update that allows two devices with the same public IP address to access Plutonium? That would be amazing. Thanks to DrSkum88 and @El-PajaMortal for your replies; your questions and suggestions helped me reach this conclusion.

@El-PajaMortal said:

ztryithard Use a private network in RadminVPN

Thanks for the suggestion. I tried using ProtonVPN, and it works to log into the game, but when I try to join a room, first of all, I usually get kicked out, and second, my wife and I can't join the same room. When we're both logged in, some rooms appear for me and other rooms appear for her, and when I try to join her room or vice versa, I get an error message saying I can't join.

DrSkum88 said:

Can you ping each other? Can you use nmap to ensure there isn't any firewall rules blocking you?
Also you can try this:

Thanks for the reply. Good point—I just realized that our computers aren't pinging each other. I'll fix that and see if it works. As for JugAndDoubleTap, we can give it a try, but we'd prefer to play online with other friends. I'll run the necessary tests and let you know if it worked or not.