Ok, cheers. Your setup makes more sense now. Haven’t used Proxmox myself (though I’ve heard good things). I’ve only had basic exposure to ESXi and Server 2016 clustering.
As to what actually happens in terms of clients joining a server, I haven’t a clue. My networking knowledge is probably the weakest part for me - I’m mostly a hardware guy - and my Linux isn’t a lot better. I can just about manage moving around directories and changing firewall rules via terminal. 
Since it seems like things are working on the Network layer but don’t get to Transport, maybe it’s the port rules? Have you tried simply restarting iptables? I doubt that’s it to be honest, but who knows? Easy fix if it is.