@Maholmire From the server's side, make sure that that port isn't filtered (I don't know what NixOS does with /etc/hosts.{allow,deny} or iptables or if it uses ufw) and that from inside, pinging the outside works (so that you can verify your routes work), but this is as much as I can say because I have not used and do not recommend either Virtualbox or NixOS and if you're already in a VM, maybe just put it in the VM instead of putting a VM in the VM.